Dc-dc converter

【0001】

[Technical field to which the invention belongs]

The present invention relates to an isolated DC-DC converter.

【0002】

[Background technology]

Figure 4 shows an example of a circuit configuration of a DC-DC converter. The DC-DC converter shown in FIG. 4 is a forward converter, which is disclosed in Patent Document 1 (Japanese Unexamined Patent Publication No. 2001-161062).

【0003】

The DC-DC converter shown in FIG. 4 includes a transformer 2, a latch circuit 25, a PWM modulator 26, a start circuit 27, an output rectification smoothing circuit 28, and an auxiliary rectification smoothing circuit 29. ing.

【0004】

Further, the transformer 2 includes a primary coil 2a, a secondary coil 2b, and an auxiliary coil 2c. The latch circuit 25 includes a PNP transistor 19, resistors 20, 21, 22, an N-channel MOSFET 23, and a capacitor 24. The PWM modulation unit 26 includes a main switch 3 and a PWM control IC 4 which is a control circuit for switching operation of the main switch 3. The PWM control IC 4 has a power supply terminal Vcc and a gate drive pulse output. It has a terminal OUT, an operation control input terminal OFF, and a ground terminal GND. The start-up circuit 27 includes an N-channel MOSFET 5, resistors 6 and 7, and a Zener diode 8. The output rectifying / smoothing circuit 28 includes diodes 9 and 10, a choke coil 11, and a capacitor 12. The auxiliary rectifying and smoothing circuit 29 includes diodes 14 and 15, a choke coil 16, and a capacitor 17.

【0005】

An operation example of this DC-DC converter will be described. For example, from the gate drive pulse output terminal OUT of the PWM control IC 4, an on signal for turning on the main switch 3 and an off signal for turning off the main switch 3 are alternately output toward the main switch 3. The main switch 3 performs an on / off switching operation based on the on signal and the off signal. By the switching operation of the main switch 3, the DC voltage input from the external DC input power supply 1 is converted into an AC voltage on the primary coil 2a side of the transformer 2 and transmitted to the secondary coil 2b side of the transformer 2. The output rectifying and smoothing circuit 28 rectifies and smoothes the AC voltage output from the secondary coil 2b of the transformer 2 and converts it into a DC voltage. This DC voltage Vout is supplied to the external load device 13 connected to the DC-DC converter. The signal corresponding to this output voltage Vout is transmitted to the PWM control IC 4 as a feedback signal by a feedback loop (not shown). Based on this feedback signal, the PWM control IC 4 controls the switching operation of the main switch 3 by the PWM control method, so that the DC voltage Vout supplied to the load device 13 is stabilized.

【0006】

Since the auxiliary rectifying smoothing circuit 29 has the same choke input rectifying type circuit configuration as the output rectifying smoothing circuit 28, both the choke coil 11 of the output rectifying smoothing circuit 28 and the choke coil 16 of the auxiliary rectifying smoothing circuit 29 are in the current continuous mode. DC voltage V output from the auxiliary rectification smoothing circuit 29 when₂₉Is almost proportional to the output voltage Vout. Therefore, the DC voltage V is controlled by the stabilization control of the output voltage Vout by the PWM control IC4.₂₉Is almost stable. This DC voltage V₂₉Is supplied to the power supply terminal Vcc of the PWM control IC4 as the power supply voltage of the PWM control IC4. If the output voltage Vout drops due to a short circuit of the load, etc., the DC voltage V₂₉Also decreases almost proportionally.

【0007】

The start-up circuit 27 is driven when the power supply voltage is not supplied from the auxiliary rectification smoothing circuit 29 to the power supply terminal Vcc of the PWM control IC 4, or when the power supply voltage is very low, for example, at the time of start-up or when the load is short-circuited. It is a circuit. That is, in this start-up circuit 27, the gate of the N-channel MOSFET 5 is connected to the cathode of the Zener diode 8, and a current flows from the DC input power supply 1 to the Zener diode 8 via the resistor 7 to cause the N-channel MOSFET 5 to flow. The gate voltage is fixed to the Zener voltage with respect to the ground. The source of the N-channel MOSFET 5 is connected to the output section of the auxiliary rectification smoothing circuit 29, and the source of the N-channel MOSFET 5 is the DC voltage V output from the auxiliary rectification smoothing circuit 29.₂₉Is applied.

【0008】

For example, DC voltage V from the auxiliary rectification smoothing circuit 29 as at startup.₂₉Is not output, or DC voltage V₂₉Is very low, or the DC voltage is V due to a load short circuit, etc.₂₉DC voltage V due to a decrease in₂₉The N-channel MOSFET 5 conducts when the difference between the voltage and the Zener voltage (that is, the gate-source voltage of the N-channel MOSFET 5) extends beyond the threshold voltage. As a result, a current flows from the DC input power supply 1 to the power supply terminal Vcc of the PWM control IC 4 via the resistor 6.

【0009】

In the normal operation of the DC-DC converter, the N-channel MOSFET 5 is turned off and the power supply voltage is supplied from the auxiliary rectification smoothing circuit 29 to the power supply terminal Vcc of the PWM control IC 4, but at startup or when the load is short-circuited. As described above, when the voltage supplied from the auxiliary rectification smoothing circuit 29 to the power supply terminal Vcc of the PWM control IC 4 is zero or very low, the N channel MOSFET 5 of the start circuit 27 becomes conductive as described above. The starting circuit 27 supplies a voltage based on the input DC power supply 1 to the power supply terminal Vcc of the PWM control IC 4 as a power supply voltage.

【0010】

The latch circuit 25 is a circuit that performs a low voltage protection operation, and is in a state where the N-channel MOSFET 5 is conducting (that is, the output voltage V of the auxiliary rectification smoothing circuit 29).₂₉Is lower than the set voltage for low voltage protection operation), but if it continues for a certain period of time or longer, the DC-DC converter is latched and stopped. That is, when the N-channel MOSFET 5 conducts, a voltage is generated across the resistor 6 and a base current flows through the resistor 20 to the PNP transistor 19, whereby the PNP transistor 19 is turned on. When the PNP transistor 19 is turned on, a current flows from the DC input power supply 1 to the capacitor 24 via the resistor 21, and the capacitor 24 is charged according to the time constant determined by the resistor 21 and the capacitor 24. Since the charging voltage of the capacitor 24 is applied to the gate of the N-channel MOSFET 23, the N-channel MOSFET 23 is turned on when the charging voltage of the capacitor 24 reaches the threshold voltage of the N-channel MOSFET 23. As a result, the voltage at which the operation control input terminal of the PWM control IC4 is OFF becomes low level.

【0011】

When the voltage of the operation control input terminal OFF becomes low level in this way, the output of the ON signal (gate drive pulse) from the PWM control IC 4 to the main switch 3 stops. By stopping the output of this on signal, the main switch 3 remains off, and the voltage Vout output from the DC-DC converter to the load device 13 decreases. In addition, the voltage V output from the auxiliary rectification smoothing circuit 29.₂₉However, since the output voltage drops like Vout, the conduction state of the N-channel MOSFET 5 of the start circuit 27 is maintained. As a result, the PNP transistor 19 and the N-channel MOSFET 23 are kept on, and the DC-DC converter is latched off.

【0012】

Although the PNP transistor 19 is turned on at startup, the time constants of the resistor 21 and the capacitor 24 are set longer than the time until the startup operation is completed, which leads to the latch stop at startup. Absent. On the other hand, when the short-circuit state of the load continues longer than the time constant of the resistor 21 and the capacitor 24, the DC-DC converter is latched and stopped by the latch circuit 25 as described above.

【0013】

[Patent Document 1]

Japanese Unexamined Patent Publication No. 2001-161062 [0014]

[Problems to be Solved by the Invention]

By the way, when the voltage Vout output from the output rectifying and smoothing circuit 28 drops due to, for example, a load short circuit, the drop in the output voltage Vout is transmitted to the PWM control IC4 by a feedback loop (not shown), and the PWM control IC4 Therefore, the switching control of the main switch 3 is performed in order to compensate for the decrease in the output voltage Vout. As a result, the output voltage Vout rises and tries to stabilize at the set voltage.

【0015】

At this time, even if a load short circuit occurs, the output voltage V of the auxiliary rectification smoothing circuit 29 due to the load short circuit occurs.₂₉(In other words, when the load is short-circuited, the output voltage Vout of the output rectification smoothing circuit 28 and the output voltage V of the auxiliary rectification smoothing circuit 29₂₉When the switching operation of the main switch 3 is performed to compensate for the decrease in the output voltage Vout, the output voltage of the auxiliary coil 2c rises. As a result, the output voltage V of the auxiliary rectification smoothing circuit 29₂₉Phenomenon occurs. In recent years, there has been a demand for higher output voltage Vout, and when component constants and the like are designed to meet this demand, the auxiliary rectification smoothing circuit 29 when a load short circuit occurs as described above is caused by this. Output voltage V₂₉The upward trend of

【0016】

By the way, the PWM control IC has a lower withstand voltage (that is, the difference between the voltage level of the on signal output from the PWM control IC toward the main switch and the withstand voltage upper limit voltage of the PWM control IC is smaller) is higher. The accuracy of switching control of the main switch is better than that of the withstand voltage. For this reason, a PWM control IC 4 with a low withstand voltage may be used. The output voltage V of the auxiliary rectification smoothing circuit 29 when a load short circuit occurs due to the use of this low withstand voltage PWM control IC and the recent increase in voltage.₂₉The following problems have come to occur in relation to the large upward trend of.

【0017】

That is, the output voltage Vout of the output rectifying and smoothing circuit 28 and the output voltage V of the auxiliary rectifying and smoothing circuit 29, as in the case of a load short circuit.₂₉The proportional relationship with is broken, and the output voltage V₂₉Is uncontrolled and the output voltage V₂₉If the voltage rises unnecessarily, the withstand voltage of the PWM control IC4 is low, so its large output voltage V₂₉There is a problem that the voltage derating of the PWM control IC4 cannot be secured with respect to (power supply voltage). Also, the power supply voltage V₂₉However, there is a problem that a situation in which the withstand voltage upper limit voltage of the PWM control IC4 is easily exceeded is likely to occur.

【0018】

In order to solve this problem, a high withstand voltage PWM control IC may be used, but the accuracy of the switching control of the main switch 3 is worse than that of the low withstand voltage PWM control IC.

【0019】

In addition, the following problems also occur. That is, in the DC-DC converter of FIG. 4, the voltage V output from the auxiliary rectification smoothing circuit 29.₂₉Is supplied to the PWM control IC 4 as a power supply voltage and is supplied to the latch circuit 25 as a detection voltage of the output voltage Vout. The latch circuit 25 is the output voltage V of the auxiliary rectification smoothing circuit 29.₂₉The latch stop operation is performed based on. Further, the voltage level of the on signal applied from the PWM control IC 4 to the main switch 3 is a voltage level equal to the power supply voltage supplied to the PWM control IC 4. Furthermore, the threshold value of the main switch 3 tends to increase as the environmental temperature decreases. From these facts, the voltage when the latch circuit 25 performs the low voltage protection operation (that is, the set voltage for the low voltage protection operation) is set so that the DC-DC converter can be reliably started even in a low temperature environment. , It is necessary to set the value higher than the threshold value of the main switch 3 in a low temperature environment.

【0020】

In this case, since the withstand voltage upper limit voltage of the PWM control IC 4 is low, the voltage difference between the withstand voltage upper limit voltage of the PWM control IC 4 and the set voltage for the low voltage protection operation becomes small. Since the set value of the power supply voltage input to the PWM control IC 4 is set within the voltage range from the set voltage for the low voltage protection operation to the withstand voltage upper limit voltage of the PWM control IC 4, it is lower than the withstand voltage upper limit voltage of the PWM control IC 4. When the voltage difference from the set voltage for voltage protection operation becomes small, the voltage difference between the power supply voltage set in the PWM control IC4 and the withstand voltage upper limit voltage and the power supply voltage set in the PWM control IC4 and the low voltage protection operation inevitably occur. The voltage difference from the set voltage for is small. Therefore, the power supply voltage V input to the PWM control IC4₂₉Even if the voltage fluctuates within the range considered normal, the power supply voltage V₂₉Exceeds the withstand voltage upper limit voltage of PWM control IC4, or the power supply voltage V₂₉Is lower than the set voltage for the low voltage protection operation, the latch circuit 25 operates, and the DC-DC converter stops latching, which may cause a malfunction of the low voltage protection operation.

【0021】

To solve this problem, a high withstand voltage PWM control IC is used to increase the withstand voltage upper limit voltage of the PWM control IC 4, so that the withstand voltage upper limit voltage of the PWM control IC 4 and the set voltage for low voltage protection operation are set. It is conceivable to widen the voltage difference, but the accuracy of the switching control of the main switch 3 will be worse than when using a low-voltage PWM control IC.

【0022】

The present invention has been made to solve the above problems, and an object of the present invention is to improve the accuracy of switching control of a main switch by using a low withstand voltage control circuit and to improve the reliability of low voltage protection operation. The purpose is to provide a DC-DC converter that can be used.

【0023】

[Means for solving problems]

In order to achieve the above object, the present invention has the following configuration as a means for solving the above-mentioned problems. That is, in the present invention, AC power is output from the secondary side of the transformer by the switching operation of the main switch provided on the primary side of the transformer, and the AC power is connected to the secondary side of the transformer for output rectification smoothing. In a DC-DC converter equipped with a configuration that is converted to DC by a circuit, a control circuit that controls the switching operation of the main switch and an auxiliary that rectifies and smoothes the AC output power of the auxiliary coil provided in the transformer and converts it to DC. The rectifying and smoothing circuit, the control power supply path that connects the output unit of this auxiliary rectifying and smoothing circuit, and the control circuit, and the output voltage of the auxiliary rectifying and smoothing circuit are regarded as the output voltage of the output rectifying and smoothing circuit and detected, and the detected voltage is A latch circuit is provided in which the switching operation of the main switch is stopped and the DC-DC converter is latched and stopped when it is detected that the voltage has dropped below the set voltage for the low voltage protection operation. Is equipped with a series regulator that generates the power supply voltage set in the control circuit using the voltage output from the auxiliary rectifying and smoothing circuit and supplies it to the control circuit, which is caused by the rise in the output voltage of the auxiliary rectifying and smoothing circuit. Therefore, it is characterized in that a power supply voltage higher than the withstand voltage upper limit voltage of the control circuit is prevented from being supplied to the control circuit.

【0024】

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, examples of embodiments according to the present invention will be described with reference to the drawings.

【0025】

An example of an embodiment of the DC-DC converter according to the present invention is shown in FIG. The DC-DC converter of this embodiment has a series regulator 18 in addition to the configuration shown in FIG. In the DC-DC converter of this embodiment, the configuration other than the configuration related to the series regulator 18 is almost the same as the configuration shown in FIG. In this embodiment, the same components as those of the circuit shown in FIG. 4 are designated by the same reference numerals, and duplicate description of the common parts will be omitted.

【0026】

The series regulator 18 is provided in the control power supply path 30 that connects the output unit of the auxiliary rectification smoothing circuit 29 and the power supply terminal Vcc of the PWM control IC 4. This series regulator 18 has a voltage V output from the auxiliary rectification smoothing circuit 29.₂₉It has a configuration that generates a power supply voltage set for the PWM control IC4 by using the above and supplies the voltage to the power supply terminal Vcc of the PWM control IC4. Therefore, the output voltage V of the auxiliary rectification smoothing circuit 29₂₉Even if the voltage rises, the set power supply voltage is supplied from the series regulator 18 to the power supply terminal Vcc of the PWM control IC 4, so the output voltage V of the auxiliary rectification smoothing circuit 29₂₉It is possible to prevent a situation in which a power supply voltage higher than the withstand voltage upper limit voltage is applied to the PWM control IC 4 due to an increase in the voltage.

【0027】

Further, the output voltage of the series regulator 18 is applied to the source of the N-channel MOSFET 5 involved in the latch stop operation of the latch circuit 25. The series regulator 18 has an output voltage V of the auxiliary rectification smoothing circuit 29.₂₉When the voltage drops below the set power supply voltage Vcc, the output voltage of the series regulator 18 becomes the output voltage V.₂₉It is a configuration that decreases as the amount of Therefore, in the latch circuit 25, when the output voltage Vout of the output rectifying / smoothing circuit 28 drops to a extent that low voltage protection operation is required, the output voltage V of the auxiliary rectifying / smoothing circuit 29 accompanies the drop of the output voltage Vout.₂₉The decrease in the voltage can be detected by the output voltage of the series regulator 18, and the latch stop operation can be performed.

【0028】

The present invention is not limited to the configuration of this embodiment, and may take various embodiments. For example, in the embodiment, the auxiliary rectifying and smoothing circuit 29 has a circuit configuration in which the auxiliary rectifying and smoothing circuit 29 is connected to the auxiliary coil 2c of the transformer 2 to rectify and smooth the output voltage of the auxiliary coil 2c. A secondary coil is provided in the choke coil 11, and the auxiliary rectification smoothing circuit 29 is connected to the secondary coil of the choke coil 11 to peak rectify and smooth the output voltage of the secondary coil during the off period of the main switch 3. It may be a circuit configuration to be used. Also in this case, a voltage substantially proportional to the output voltage Vout of the output rectifying / smoothing circuit 28 can be output from the auxiliary rectifying / smoothing circuit 29.

【0029】

Further, in the embodiment, the latch circuit 25 is configured to perform the latch stop operation by using the output voltage of the series regulator 18, but for example, as shown in FIG. 2, the output of the auxiliary rectification smoothing circuit 29. The unit and the source of the N-channel MOSFET 5 involved in the latch stop operation are directly connected, and the latch circuit 25 directly detects the output voltage V of the auxiliary rectification smoothing circuit 29.₂₉The latch stop operation may be performed based on the above. In this case, the output voltage of the series regulator 18 is a voltage dedicated to the power supply voltage of the PWM control IC 4.

【0030】

Further, the voltage obtained by peak rectifying the voltage of the choke coil 16 of the auxiliary rectifying smoothing circuit 29 during the off period of the main switch 3 is the output voltage V of the auxiliary rectifying smoothing circuit 29.₂₉The voltage is almost equal to. For this reason, for example, a circuit for peak rectifying the voltage of the choke coil 16 of the auxiliary rectification smoothing circuit 29 during the off period of the main switch 3 is provided, and the latch circuit 25 uses the peak rectification circuit of the auxiliary rectification smoothing circuit 29. Output voltage V₂₉May be indirectly detected and the latch stop operation may be performed based on the detected voltage.

【0031】

The latch circuit 25 is the output voltage V of the auxiliary rectification smoothing circuit 29.₂₉Is not detected via the series regulator 18, but the output voltage V of the auxiliary rectification smoothing circuit 29.₂₉May be indirectly detected by other means, or may be directly detected, and the latch stop operation may be performed based on the detected voltage.

【0032】

Further, for example, as shown in FIG. 3, an external control terminal 31 for inputting a command signal for stopping the switching operation of the main switch 3 from the outside is provided, and the external control terminal 31 is provided from the outside. A configuration may be provided in which a command signal for stopping the switching operation of the main switch 3 can be applied to the DC-DC converter via the system. In this case, it is preferable to provide the latch stop prevention circuit 32. When the latch stop blocking circuit 32 detects that a command signal for stopping the switching operation of the main switch 3 is applied from the outside via the terminal 31 for external control, the switching operation of the main switch 3 according to the command is performed. It has a configuration in which the latch circuit 25 does not perform the latch stop operation even if the output voltage Vout of the output rectifying smoothing circuit 28 drops due to the stop.

【0033】

Further, for example, as shown by the chain line in FIG. 3, a switching stop circuit 33 may be provided when the input power supply is lowered. The switching stop circuit 33 when the input power supply is lowered has a configuration in which the switching operation of the main switch 3 is stopped when the voltage input from the DC input power supply 1 is detected and the detected voltage is detected to be lower than the set voltage. It is a thing. Even when the switching stop circuit 33 when the input power supply is lowered is provided, it is preferable to provide the same latch stop prevention circuit 32 as described above. That is, in this case, when the latch stop prevention circuit 32 detects that the switching operation of the main switch 3 has stopped by the switching stop circuit 33 when the input power supply is low, the output rectification is performed by stopping the switching operation of the main switch 3. It has a configuration in which the latch circuit 25 does not perform the latch stop operation even if the output voltage Vout of the smoothing circuit 28 drops.

【0034】

Further, in the embodiment, the DC-DC converter is of the forward converter type, but the present invention can be applied to, for example, a DC-DC converter of another power conversion circuit type such as a flyback converter type. is there.

【0035】

[Effect of the invention]

According to the present invention, a series regulator is provided in the control power supply path connecting the auxiliary rectification smoothing circuit and the control circuit. Therefore, even if the proportional relationship between the output voltage of the output rectifying and smoothing circuit and the output voltage of the auxiliary rectifying and smoothing circuit is broken due to a load short circuit and the output voltage of the auxiliary rectifying and smoothing circuit rises abnormally, the series regulator The set power supply voltage is supplied to the control circuit. This makes it easier to secure the voltage derating of the control circuit with respect to the power supply voltage input to the control circuit.

【0036】

In addition, since the series regulator suppresses fluctuations in which the power supply voltage input to the control circuit rises above the set value, for example, the set value of the power supply voltage is set to a voltage closer to the withstand voltage upper limit voltage of the control circuit than before. Even so, it is possible to avoid a situation in which a large power supply voltage exceeding the withstand voltage upper limit voltage is applied to the control circuit. Therefore, by setting the set value of the power supply voltage to a higher voltage near the withstand voltage upper limit voltage of the control circuit than before, the set value of the power supply voltage and the low voltage protection operation when the latch circuit stops the latch. It is possible to widen the voltage difference from the set voltage for. As a result, it is possible to prevent a malfunction of the low voltage protection operation in which the latch circuit performs the latch stop operation even though the power supply voltage is a drop fluctuation within a normal range. Therefore, the reliability of the low voltage protection operation can be improved.

【0037】

In particular, a high-precision, low-voltage control circuit may be used to improve the accuracy of switching control of the main switch. In this case, the set value of the power supply voltage and the set voltage for low-voltage protection operation are used. Since the voltage difference between the voltage and the voltage is small, a malfunction of the low voltage protection operation is likely to occur, which has been a problem. However, by providing the unique configuration of the present invention, the problem can be easily solved. From this, the unique configuration of the present invention is very effective when a low withstand voltage control circuit is used.

【0038】

By the way, if the output voltage of the output rectifying / smoothing circuit is directly detected and the low voltage protection operation is performed, a photocoupler is used. Photocouplers have the disadvantages that they have a limited life and cannot be used in a high temperature environment. On the other hand, in the present invention, the output voltage of the output rectifying and smoothing circuit is indirectly detected by using the auxiliary rectifying and smoothing circuit, and the low voltage protection operation is performed based on the detected voltage. Do not use. Therefore, it is possible to suppress the occurrence of problems caused by the photocoupler, and it is possible to realize high reliability of the DC-DC converter.

【0039】

Even when the latch circuit that performs the low voltage protection operation directly detects the output voltage of the auxiliary rectification smoothing circuit and performs the latch stop operation based on the detected voltage, the output voltage of the auxiliary rectification smoothing circuit is determined. The same latch stop operation can be performed even in the case of a configuration in which the latch stop operation is indirectly detected via the series regulator and the latch stop operation is performed based on the detected voltage. The same excellent effect as described above can be obtained by providing the configuration peculiar to the present invention regardless of the detection method of the detection voltage taken into the latch circuit from the output unit of the auxiliary rectification / smoothing circuit.

【0040】

If a terminal for external control is provided and a latch stop prevention circuit is provided, operation control can be performed from the outside of the DC-DC converter while avoiding latch stop by the latch stop prevention circuit. Has the effect of

【0041】

If the switching stop circuit is provided when the input power supply is low and the latch stop prevention circuit is provided, the input voltage supplied from the outside to the DC-DC converter is set by the switching stop circuit when the input power supply is low. When the voltage drops below the voltage of, the switching operation of the main switch is stopped and the operation of the DC-DC converter can be stopped. Moreover, when the switching operation of the main switch is stopped due to such an abnormality of the input voltage, the latch stop can be prevented by the latch stop blocking circuit, so that the restart after the input voltage is recovered becomes easy.

[Simple explanation of drawings]

FIG. 1 is a circuit diagram showing an example of an embodiment of a DC-DC converter according to the present invention.

FIG. 2 is a circuit diagram showing another embodiment.

FIG. 3 is a circuit diagram showing another embodiment.

FIG. 4 is a circuit diagram showing an example of a conventional DC-DC converter.

[Explanation of symbols]

1 DC input power supply 2 Transformer 3 Main switch 4 PWM control IC 5,23 N channel MOSFET 6,7,20,21,22 Resistor 8 Zener diode 9,10,14,15 Diode 11,16 Choke coil 12,17,24 Capacitor 13 Load device 18 Series regulator 19 PNP transistor 25 Latch circuit 26 PWM modulator 27 Start circuit 28 Output rectification smoothing circuit 29 Auxiliary rectification smoothing circuit