Fig. 1 shows a simplified schematic of a typical flyback converter topology. Not shown is the feedback loop components and the source of VCC power for the PWM IC, which will not be discussed in this article. A brief description of the circuit is as follows: V1 is the main source of AC power. The AC power is rectified by the diodes D1 - D4 and filtered into a DC voltage by the input capacitor C1. L1 and L2 make up the switching transformer, with L1 being the primary and L2 being the secondary (isolated form the primary side high voltage.) There can be several secondaries with either a low voltage output of high voltage output or combination of both. M1 is a NMOS mosfet usually used for switching current through L1. This semiconductor could also be an IGBT or other type of high frequency solid state switch. R1 senses the peak switch current and allows the PWM IC to turn off the M1 switch if the current reaches the maximum setting allowed determined by the value of R1 and the voltage threshold level in the IC, usually approximately 1 volt. When the switch M1 turns off, the current in the inductance L1 causes a high voltage spike to appear on the drain of M1 which is due to the leakage inductance of the transformer. The best made switching transformers usually have less than 3% leakage inductance between the primary and secondaries. The voltage spike is "snubbed" by the diode D5 and the snubbing elements R2 and C2. R2 will dissipate power during the snubbing period and so can have a major effect on the efficiency of the converter. Following the "spike", energy is transferred to the secondaries, rectified by D6 and filtered by C3. R3 represents the load. A feedback signal is usually taken from one of secondaries to feedback to the PWM IC. The feedback components are not shown in Fig. 1. There are many "gotchas" in a design like this if the designer is not careful in his selection of circuit values and components. This article will start with a few basic calculations to get started.
Required input power is calculated from the required output power as
Pin = Po / Eff
where Po is the required output power and Eff is the required efficiency of the converter. These values are usually in the unit specifications.
Minimum and maximum DC voltage after rectification (ignoring losses in the diodes and traces, etc.)
Vdcmin = 2^0.5 * Vacmin
Vdcmax = 2^0.5 * Vacmax
An important calculation is the reflected voltage seen at the switch M1 from the secondary of the transformer:
VRO = Np * (( Vo + Vf ) / Ns )
where Np is the number of primary turns and Ns is the number of secondary turns (see my previous posts on transformer design), and Vf is the diode forward voltage.
We can make a calculation to see if the design will work with the minimum AC voltage input from the following equation:
Vdcmin = ( 2 * Vacmin^2 - Pin * ( 1 - Dch ) /( C1 * Fl ) ) ^0.5
where Dch is the DC link charging ratio (refer to Fairchild semiconductor application note AN-4138) Dch is usually approximately 0.2 for a quick estimate. Fl is the AC line frequency. If Vdcmin comes out to be an imaginary number this means that Vacmin is too low, or Pin is too high, or C1 is too low, Dch is too high, or all of the above.
The nominal voltage on the drain of the switch (not including the leakage inductance spike) is
Vdsnom = Vdcmax + VRO
The base value of the peak current in L1 and M1 is given by
IEDC = Pin / (Vdcmin * Dmax )
where Dmax is the maximum allowed duty cycle on switch on-time. This value is usually determined by the specifications of the PWM IC but it is also affected by the current limit set by R1 in the circuit.
An additional component of the current in continuous current mode (CCM) is given by
dI = Vdcmin * Dmax / (L1 * Fs )
where Fs is the switching frequency determined by the PWM IC and sometimes some external RC timing components (not shown if Fig.)
The maximum peak current is then
Idspeak = IEDC + ( dI / 2 )
We can also calculate the RMs switch current in L1 and M1. This information is good to determine so that switch and transformer power dissipation can be calculated.
Idsrms = ( 3 * IEDC^2 + dI^2 * (Dmax / 12 ) ) ^0.5
Idsrms is the RMS value of switch current at minimum DC voltage input.
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