Single-transistor forward converters are popular in higher-power isolated power supplies because they feature low parts count and component stress. On the downside, careful attention must be given to transformer reset to prevent saturation and excessive voltages on power devices. In practice, the feedback loop and cycle-by-cycle current limit are relied on to respond to various normal and abnormal conditions by adjusting the duty cycle. This should ensure that the correct volt-seconds are applied to the transformer at all times—but there are no set duty-cycle limits.
One way to limit duty cycle is to use a controller IC with an adjustable maximum duty cycle. Disadvantages of this method include a limited number of controller choices and a lack of protection against excessive duty at higher input voltages. A better solution is to add a simple transistor comparator, as shown in the figure. The circuit, Q1 and associated components, can be added to virtually any controller and acts as an independent volt-second clamp.
Operation is straightforward. The switching cycle begins with the primary gate voltage (VGATE) going high. This reverse-biases Q2's collector-base junction, which is being used as a diode. With Q2 off, C1 begins charging with a current determined by the input voltage and the value of R1. In normal operation, C1 is discharged by Q2 when VGATE goes low at the end of the main switch's on time, and the circuit has no effect on the operation of the supply. If, at any input voltage, VGATE remains on too long, the voltage on C1 exceeds U1's 3-V reference (plus Q1's VBE) and Q1 pulls up on ISENSE, thereby terminating the switch's on time immediately.
The collector-base junction of Q2 was used because it roughly matches Q1's VBE and can withstand the 12 V available on VGATE. D1 and R2 keep U1's bipolar output stage from dipping below ground due to current drawn by Q3's drain-to-gate capacitance following the peak of the drain voltage during transformer reset. This ensures that C1 is discharged to the same level every time.
The LT1725 controller was chosen because it has unique sampling feedback circuitry designed for flyback converters. Adding the transistor comparator reduces the IC's fixed 90% maximum duty cycle, and an over-winding on the output inductor supplies feedback and primary bias power. The transformer and output inductor give 1500 V of isolation. With the configuration shown, the supply provides +12 V at 3 A and −12 V at 1 A. Windings can also be configured for a ±2-A, or a single 12-V/4-A supply. Full-load efficiency remains above 88% over the 36- to 72-V input range. Line/load regulation is ±3.5% including asymmetric loading.