Defeating turn on overshoot

For this discussion, we restrict our analysis to a forward converter. The power converter stage is shown in Figure 1 with connection points labeled that tie into the control circuit.

Figure 1. Representative Model of the Output of the Power Stage and Biasing for the Secondary Side Feedback

At some point in the turn-on process, the secondary side feedback controller takes over and sends a signal back to the primary side controller, which then controls the pulse width of the converter and regulates the output voltage.

During the startup process, a common problem is overshooting the output voltage. This is caused by the voltage feedback loop configured as an integrating controller. This is best explained with the circuit shown in Figure 2.

Figure 2. Typical Feedback Circuit with Optocoupler Primary/Secondary Isolation

Components R1, R2, C1, R3, C2, R4 and R5 were chosen during the loop analysis stage of the design to satisfy the control loop requirements during operation. At start up conditions are different.

Prior to turn-on, VOUT and Vbias are both at ground. However, since Vbias is powered with the first pulses from the PWM (see Figure 1), the Vbias reaches its design voltage almost immediately. The voltage for VOUT, on the other hand, has to come through an inductor. This limits the rate of rise of the output voltage, VOUT.

Initially, the voltage at the junction of R4/C2 is at ground " before the power is applied, as is the voltage at the R1/R2 junction. This establishes an initial condition across all the capacitors of zero volts.

Page 2: During startup
Page 1 | 2 | 3 | 4 | 5