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LM3445 Transformer Design Procedure

WADE NOVIN
Prodigy 100 points
Other Parts Discussed in Thread: LM3445

The design for an isolated driver using the LM3445 requires a discontinuous mode flyback transformer.

However, in treatments of this topology, there is usually a DC voltage specified that has a minimum and maximum value.

In designs and application notes for the LM3445, the schematic usually shows the rectified AC voltage feeding the primary directly without filtering bulk capacitors

So what is the assumed input voltage to the primary in this scenario, or minimum and maximum voltages?

In general, given the output power, how is the primary side inductance calculated?

At an LED power rating of 90 Watts, such that the LED voltage is ~38V @ 2.4 Amps would bulk storage capacitors  be necessary?

Thanks in advance for any advice.

Wade

  • 0
    TI__Mastermind 28655 points

    Hello Wade,

    The input to the power conversion is rectified AC.  There is a small capacitor after the rectifier bridge to support the switching energy but the idea is to take the input as rectified AC and try to make the power conversion draw current from the source as a sinewave in phase with the voltage.  This is power factor correction which is trying to emulate a resistive load.  If there was a large bulk capacitor on the input side it wouldn't be possible to pull current from the source except at the peak of the rectified AC.  So to answer, the input voltage is 0- to peak of the rectified AC, 169 volts for 120 VAC, 325 volts for 230 VAC (0-169 or 0-325).

    The reference for the current regulator on the LM3445 EVM is a resistor divider to filter 2 pin.  This is how the LM3445 tries to provide power factor correction.  Input current will be sinusoidal in phase with the rectified AC.

    How I would start the calculations, though it's a bit more complicated than this, would be at the peak of the rectified AC.  That is where the current will be the highest so the off time current will be present the longest and needs to be less than the off-time of the LM3445 if it is to be truely discontinuous.  The power delivered to the load at this point would be two times higher than the required power (2 * 90W / efficiency).  This is because the input voltage is 1.414 * Vrms and the input current is around 1.414 Irms (peak of the rectified AC).  This would be true with a 90 watt incandescent bulb as well, 170 watts at the peak of the rectified AC..  The on-time can be calculated with V=Ldi/dt.  The off time as well by either using the reflected voltage to the primary and primary inductance or the output voltage and secondary inductance.  In all this a turns ratio needs to be chosen.  This will determine several things.  One will be the discharge rate of the flyback transformer (on time versus transformer discharge time).  Quick example would be at 169 peak volts and an output of 38 volts, if the turns ratio was 169/38 then the on-time and discharge time would be about the same (I'm neglecting diode drops and other things).  Another important consideration would be reflected voltage to the switching MOSFET.  Example the 120 volt EVM has a 4:1 ratio with an LED output of a maximum of 27 volts which makes the reflected voltage to the primary 4*27 volts for the highest stack.  This means at the peak of the rectified AC the MOSFET will be on less than the secondary side diode conducts.  On time is 169V=L*di/dt, discharge time is 4*27V=L*di/dt, L is the same for both since reflected voltage is being used.

    The bulk capacitance exists on these designs but it's on the secondary side.  Without it there will be 100% ripple in the LEDs and it can cause damage to the LEDs since the peak current at the peak of the rectified AC will be much higher than the average current.  Also, the sizing of the capacitors is related to the 100 or 120 Hz 1/2 sine wave rate since it is power factor corrected.

    Thanks,