I am making a 12 V battery charger getting power from 220V AC. The topology is Half bridge with capacitor voltage divider, as shown in the SG3525 datasheet.
While initial testing on vero-board, The voltage at the midpoint of half bridge remains high after the pulse of the high MOSFET and remains low after the pulse of low MOSFET. This makes it a 50% square-wave regardless of the duty cycle from SG3525A. A similar square wave appears on the secondaries.
I want to have control on output voltage and current, so need to have duty cycle in my (SG3525A's) hand.
Can anyone figure out what's causing the problem??
The specs I am working with are as follows.
Vin: 220 VAC, 50 Hz
Vout: 12-14 V
Iout: 0-4 A
Topology: Half bridge with forward converter
PWM controller: SG3525A
Frequency: 55 kHz
Core: #40 EE Ferrite
Primary: 60 turns of SWG25 wire
Secondary: 10+10 turns center tapped of 6xSWG25 wires
Aux: 10 turns SWG25 wire
Voltage divider capacitors: 220 uF each, with 470 kOhm in parallel to each
When testing the circuit with resistive load on Aux, 24 V DC at input and about 40% duty cycle on the SG3525A, I get the following waveforms. The Aux has just a full-wave rectifier and a 100 uF filter capacitor.
Edit: I placed a resistor in series with the primary winding of T2, so now I have the current waveforms for the primary. The voltage and current waveforms for the primary and secondary of T2, when analyzed together, may give a better picture of what's happening in the transformer.
The following diagram shows primary and secondary voltage at different duty cycles under moderate load. A resistive load is connected to the secondary, so the current is same as the voltage. The secondary voltage probe is at 10x setting.