1. Field of the Invention
The present invention is directed to isolated switching voltage converters and more particularly to primary side sensing of the output voltage on the secondary side of a switching voltage converter.
2. Description of the Related Art
Switching voltage converters (also referred to herein as switching voltage regulators) are used to provide, e.g., regulated DC output voltage from an unregulated AC input. Typical consumer products involving such switching regulators include cell phone chargers, laptop or printer power supplies (so-called “bricks”), and embedded PC power supplies.
FIG. 1 illustrates a switching regulator 100 having a flyback topology commonly used in power supplies with output power less than 200 W. The illustrated topology is suitable for discontinuous current mode (DCM) operation in which the transformer core is fully demagnetized in each cycle. Another mode of operation is continuous current mode (CCM).
The switching regulator 100 includes high voltage isolation between the DC output (secondary side) and the AC mains input (primary side). The isolation is required for safety/regulatory reasons and may also be required for functional reasons. The power is transferred from the primary side to the secondary side using a transformer 102 meeting isolation requirements and primary side high voltage switch 104.
The switching regulator shown in FIG. 1 operates as follows for DCM operation. When the power MOSFET switch 104 turns ON (TON phase) according to the gate control signal supplied by the controller integrated circuit 106, the current through primary winding 105 ramps up with a slope of Vin/Lp and the energy stored in the transformer core at the end of the TON cycle is proportional to 0.5×Lp×Ippeak2, where Lp is the transformer primary winding inductance and Ippeak is the primary winding peak current. The output current Is is zero during the TON phase and the voltage Vs is negative referenced to the secondary side ground, Vs=−Ns*Vin, where Ns is the transformer secondary/primary turn ratio.
When switch 104 turns OFF (TOFF phase), the primary inductor current Ip becomes zero and secondary current Is ramps down from the value Ispeak=Ippeak/Ns to zero, with a slope of approximately ˜(Vout+Vdout)/Ls. When Is>0 and Q1 is OFF, the output voltage is reflected according to the transformer turn ratio back to the primary side and is usually called primary flyback voltage: Vfly_p˜(Vout+Vdout)/Ns, where Vout is the output voltage, Vdout is a voltage drop across the output diode 107 and Ns is defined as above. Once the secondary winding current Is reaches zero, both transformer windings become open (provided that loading on auxiliary winding can be neglected) and flyback voltage converts to damped ringing waveform fueled by residual energy in the Lp and Cd resonant circuit, where Cd is the total equivalent capacitance at the drain of Q1.
Accurate regulation of the output voltage requires feedback proportional to output voltage. The feedback controls the duty cycle of switch 104 in order to keep the output voltage constant over changing load and input voltage. The feedback path needs to cross the isolation barrier between the primary and secondary. A common feedback solution uses an opto-coupler 108 as shown in FIG. 1.
The use of an opto-coupler for feedback has costs such as additional components as well as PCB area space. An opto-coupler feedback approach typically requires additional passive and active components like resistors, capacitors and a shunt regulator. The shunt regulator 109 can be alternatively replaced by Zener diode when lower quality output voltage regulation accuracy can be tolerated.
There have been suggestions to replace the opto-coupler and regulate the output voltage by sensing the flyback voltage at either the primary winding or at an auxiliary winding (sometimes referred to as a bias winding). Such approaches are usually denoted as regulation using magnetic flux sensing or primary side sensing. However prior art primary side sensing approaches have had problems with accurately determining the flyback voltage. Accordingly, improved primary side sensing is desirable.