Basic circuit of isolated switching power supply

The isolated switching power supply is actually a circuit deformation of the non-isolated switching power supply. Its basic working principle and energy transfer form are essentially the same. According to the difference in the specific energy transfer method, there are five basic circuits of isolated switching power supplies, such as forward and push-pull.

(1) Forward converter A forward converter is a converter that transfers input energy to the output through a transformer during the on-time of the switch.

① Single-tube forward converter. Figure 1 shows the circuit of a single-tube forward converter. The basic working principle is as follows: When the switch tube VT is turned on, the input voltage Uin is almost entirely applied to both ends of the primary coil of the transformer; the voltage induced on the secondary coil turns on VD1 and transmits the input energy to the choke Lo, capacitor Co and load. At the same time, a magnetic field is established in the transformer. When VT is turned off, VD1 is turned off, and the energy stored in Lo continues to supply power to the load through the freewheeling diode VD2. The stability of the output voltage is achieved by changing the duty cycle of the chopping pulse acting on VT.

Figure 1 Single-tube forward converter circuit

The transformer in Figure 1 has three functions: isolation, input and output. It couples the high-frequency pulses chopped by the switching transistor to the output rectification circuit; during the VT cut-off period, the magnetic field energy stored in the winding is returned to the input through the additional demagnetization coil and diode VD3. Therefore, at the beginning of each cycle, the operating point of the transformer core returns to zero.

When the limit value Uceo (breakdown voltage between collector and emitter when base is open) of the switch tube is less than 2 times Uin, in order to make it work in the safe zone, the additional circuit shown by the dotted line should be added to reduce the collector voltage and the rate of rise of the collector voltage. As soon as VT is turned off, the current in the primary coil of the transformer begins to charge the capacitor C) through the diode VD4; as soon as VT is turned on, C1 discharges through the resistor R1 to release the stored energy. In order to prevent excessive surge current, R1 should be large enough.

The advantage of the forward energy transfer circuit is that the output ripple is lower than that of the flyback type. However, when used as a multi-output converter, each output requires a secondary coil, two diodes, a choke and an electrolytic capacitor.

②Dual-tube forward converter. Figure 2 shows the circuit diagram of the dual-tube forward converter. Its working principle is similar to that of the single-tube converter, with two transistors turned on and off at the same time. When the transistor is turned off, the voltage polarity at both ends of the primary coil is reversed, and the primary current returns to the input terminal through diodes VD3 and VD4. The diodes limit the maximum collector-emitter voltage on the two transistors to the input voltage Uin. Because no additional demagnetization coil is required in the circuit, the transformer design is relatively simple; because the buffer circuit that reduces the collector-emitter voltage rise rate is eliminated, the discharge circuit has a higher efficiency. The disadvantage is that two power tubes and a relatively complex drive circuit are required.

Figure 2 Dual-switch forward converter circuit

(2) Push-pull converter A conventional push-pull converter consists of two push-pull forward converters, as shown in Figure 3. Switching tubes VT1 and VT2 work alternately, and diodes VDI and VD2 rectify the voltage converted to the secondary. After filtering, a DC voltage that meets the output requirements is obtained.

Figure 3 Conventional push-pull converter

The output power of this working mode is higher than that of the previous two single-stage circuits; the output ripple generated is lower than that of the single-stage circuit, and the ripple frequency is doubled. The voltage borne by the collector of VT1 and VT2 is twice the input voltage (if the effect of transformer leakage inductance is considered, the resulting spike voltage should also be added). The maximum duty cycle γ of the drive pulse (the pulse driving VT1 or VT2) cannot exceed 50%.

The secondary of the above circuit is rectified in full-wave center-tapped mode.