Primary Side Controller for Offline Applications
Primary regulation has been one of the main hot topics for low power battery chargers or LED applications. The common topology is flyback or buck-boost. The primary sensing technique reduces overall cost and increases the reliability of the power supply at the same time. The output of power supplies can be either constant voltage or constant current.
Primary regulation has been one of the main hot topics for low power battery chargers or LED applications. The common topology is flyback or buck-boost. The primary sensing technique reduces overall cost and increases the reliability of the power supply at the same time. The output of power supplies can be either constant voltage or constant current.
Typically for voltage regulation (constant voltage output), the auxiliary winding is used to sense the output voltage. In most cases, there is no special treatment for sensing output voltage. The voltage drop of the output diode would be included in the sensing, which causes some error in regulation because the voltage drop is depended on output current, temperature, etc. To improve the voltage sensing, the influence of voltage drop on the output diode must be eliminated. For low power applications, the flyback is operating at discontinuous current mode or boundary mode. If the controller can sense winding voltage at the moment that the current of the output diode reaches zero, the variation of voltage drop on the output diode is eliminated. Figure 1 shows the block diagram. Because of this innovative sensing technique, the output voltage regulation can be within +/-2.5%.
Figure 1: block diagram of voltage sensing
The current estimation block diagram is shown in figure 2. The diode conduction angle is detected by the auxiliary winding, and it is used to generate the control signal for the small signal switch Q1. A resistor R in series with it controls a current Vc/R, when Q1 is turned on, where Vc is the voltage developed across the capacitor Cref. When Q2 is turned off, diode D on the secondary side will conduct current. The ZCD signal will be high. The small signal FET will be turned on through the flip-flop. The capacitor Cref will be discharged through R. For the rest of the period, Q1 will be turned off, and Cref will be charged by the constant current source IREF. In steady state, the voltage of Cref will hold constant if the capacitance is large enough.
We can have the output current Iout:
This equation shows that the average output current no longer depends on the input or the output voltage, or on transformer inductance values. The external parameters defining the output current are the transformer turn’s ratio n and the current sense resistor RSENSE, both of which can be well controlled. The internal parameter tolerance of R*Iref can be guaranteed by the manufacturing trimming process.
Figure 2. Current estimation block diagram
A controller HVLED815PF is developed based on the concept described above. A high voltage MOSFET (800V) is integrated with the controller. For Vin 95VAC-132VAC, the overall output voltage regulation is within +/- 2.5%, and the current regulation is +/- 5%.
Figure 3 shows the voltage regulation and current regulation for 5V/1A throughout the load range.
Figure 3. Voltage and current regulation
The benefits of the proposed method are a low component count, compact size, and overall cost effectiveness. It is suitable for auxiliary power supplies, LED drivers, and battery charger applications.
For a more detailed implementation, please refer to a presentation by the author in Darnell’s Power Forum (DPF) 2013, now part of Darnell’s Energy Summit (DES).
