Overload protection of quasi-resonant power supply

The main characteristic of a quasi-resonant (QR) switch-mode power supply (SMPS) is the frequency variation as the input voltage changes.
For a flyback power supply, the power delivered at its output follows the following equation:

where: LP is the primary inductance, IP is the primary peak current, FSW is the switching frequency, and h is the efficiency.
Because LP and h are fixed, IP must change inversely to maintain constant power output when the switching frequency FSW changes. When the input voltage VIN increases, FSW also increases: Therefore, IP needs to be reduced accordingly as required by the feedback loop. In wide-range power applications, to achieve constant output power, the peak current almost doubles when the input voltage changes from high to low. However, QR controllers only have an overcurrent protection feature, which is part of the structural problem: the peak current is monitored and when it reaches the maximum allowed value, the controller circuit detects the overload. However, if the power supply is designed to provide the rated output power at the lowest input voltage (worst case) condition, then the power supply will have to provide more power (more than three times more than required for wide-range power applications) at higher input voltages. This is a consequence of the flyback equation.
The classic way to compensate for this effect is to create a bias on the current sense pin that compensates for the variation in peak current as a function of the input voltage VIN. This can be achieved by connecting a compensation resistor from the high voltage supply to the current sense pin (see Figure 1a): This technique is called overload protection.

Figure 1a, standard solution for OPP

However, this solution is not always possible: the CS pin may be used for another function or the pin resistance must be kept low for noise suppression, which forces a low value for the resistor in series with the current sense pin (RCS), requiring a low compensation resistor RCOMP, wasting a lot of power: This solution is unacceptable when the goal is low standby power.
To solve this problem, a portion of the input voltage can be used in order to reduce the voltage drop on RCOMP: Then, the power wasted on the resistor can be ignored.
This can be achieved using the forward voltage of the auxiliary winding: On the forward winding, there is a voltage proportional to VIN during the on-time. Most of the time, the flyback auxiliary winding already supplies power to the controller and also senses core demagnetization: By changing the configuration of the windings, it is possible to generate flyback information for demagnetization detection (during the off time) and combine it with forward information on the same winding for overload compensation (during the on time). With a diode in series with the auxiliary winding, we can obtain the forward voltage (see Figure 1b). This forward voltage is proportional to N.VIN (N is the turns ratio between the primary and auxiliary windings). RFWD is increased to provide reverse current in forward operation.

Figure 1b, practical circuit, reducing compensation losses

Knowing the value of the forward voltage and the series resistor RCS, it is relatively simple to calculate the value of the compensation resistor RCOMP required to establish the expected bias on the current sense signal at high input voltages.
The D11N4448 diode is also installed on the demonstration board of ON Semiconductor NCP1207, RCS=680Ω, RCOMP=18kΩ, RFWD=4.7kΩ, for 100Vdc protection is triggered at 60W, for 365Vdc protection is triggered at 70W, while without compensation, 100Vdc is 55W and 365Vdc is 165W (see Figure 2).

Figure 2, Maximum output power and VIN relationship provided by compensation measures applied to QR controller

Figures 3 and 4 clearly show the effect of compensation, both measured under the same load conditions. There is a large additional compensation offset at high line voltage (Figure 3), while the offset is negligible at low line voltage (Figure 4).

Figure 3, Line voltage compensation at VIN = 365Vdc


Figure 4, Line voltage compensation at VIN = 100Vdc