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Help me to understand an article on Isolated DC-DC Converter

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yongchai

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Code:
Isolated DC-DC Converter Sports Dual Outputs

Presented here is an isolated 6-W dc-dc converter optimized to provide low-cost and efficient isolated power across a 2500-V ac potential barrier. The converter, which incorporates inexpensive, readily available components, can provide either two isolated 3-W outputs (Figure 1a) or one isolated 6-W output (Figure 1b).

The CD4001, a CMOS quad NOR gate, is configured as an RC relaxation oscillator. The frequency is set at approximately:

FOSC = 0.6/(R2 × C1)
      	= 0.6/(10 kΩ × 220 pF)
      	= 273 kHz

Actual measurement of the circuit came out to 250 kHz. Resistor R3 has a value that may vary and might need to be terminated to VDD or ground. R3 is inserted to adjust the oscillator trip point so that a 50% duty-cycle waveform is available at the output of U1a and its complement is available at U1b.
 
This square wave and its complement are connected to the INA and INB inputs of the IXDD404SI, a 4-A dual-gate driver. The outputs, OUTA and OUTB of U2, are then applied to the primary of T1, a VAC #T60403-F4025-X142 transformer that has a single primary and 2500-V ac isolated, dual-secondary windings.
 
The converter is designed to function over a 13.5- to 25-V VCC power range. C5, which blocks dc from T1's primary, and C6 and C7, which filter the floating dc outputs, are all low-voltage, low equivalent-series resistance and equivalent-series inductance, multilayer ceramic capacitors. D1 and D2, which are DDS10-40BA 1-A Schottky diodes, half-wave rectify the resulting square-wave output from T1.
 
D1 and D2 are connected in an anti-parallel configuration, minimizing T1 saturation at high currents. For a single isolated output, the portion of the converter after T1's secondary is modified as shown in Figure 1b, where D1 and D2 form a push-pull output. Because of the wide VCC supply-voltage variation, a 15-V zener diode (Z1) is used to limit the voltage supplied for U1 to +15 V or less.
 
Total manufacturing volume costs for the isolated dc-dc converters are less than $3, or about $0.50/W. The total volume of the solution is about 0.25-in.3

Questions:

1) What does this sentence "C5, which blocks dc from T1's primary..." mean? Why there is 'dc' in T1's primary?

2) Would someone pls explain how D1 and D2, which are connected in an anti-parallel configuration, could minimize T1 saturation at high currents?

3) What does 'push-pull' output mean?

Many Thanks.
 

aryajur

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Re: Help me to understand an article on Isolated DC-DC Conve

I think C5 is used to block DC from the OUTA of the converter IC, not the other way around.
D1 and D2 are connected in antiparallel configuration because the way they are connected to the Transformer secondaries. If you notice the dots on the secondaries they are on different terminals, so the diodes conduct alternately. And thus it is like a push pull sort of arrangement.
 

VVV

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Re: Help me to understand an article on Isolated DC-DC Conve

First of all, push-pull can refer to an output that can both source and sink current, although some people use the term to mean an output stage with two outputs, 180° out of phase.

The circuit has two outputs, 180° out of phase. There should not be any DC across the transformer, BUT, in the real world nothing is perfect, so small differences in the saturation voltages of the internal transistors, as well as slight pulse width differences (which occur due to differences between the transistors parameters) could create a small DC component. While small, DC can easily saturate the transformer, thus the use of C5.

The diodes are not really in "antiparallel". The two original outputs of the converter have been merely connected in parallel, for higher current. This is possible because they are operated 180° out of phase, thus you have either one diode or the other conducting. The advantage is that the transformer stays the same and the diodes stay the same for higher power. sometimes this connection is referred to as "current doubling", because it effectively doubles the output current capability.
 

Engineer_Bob

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Hi, Yongchai

I agree with the explanation VVV gave, but I’m going to try to explain this to you by suggesting a way to think about it so its not really a perfectly technical explanation but I think it may be useful.

3) What does 'push-pull' output mean?

Push pull does this with the IC outputs:

Out A Out B
0 1
1 0
0 1


Its stated so that you don’t think its doing this


Out A Out B
0 1
0 0
0 1


1) What does this sentence "C5, which blocks dc from T1's primary..." mean? Why there is 'dc' in T1's primary?


Think about the core of the transformer as having the ability to store energy, but if you fill it up it will overflow. (saturate). This will appear as a big increase in the transformer primary current.

If you apply a dc voltage to the primary of a transformer it will quickly begin to draw a large current.

The capacitor is there to stop a current from flowing in the primary for too long.


2) Would someone pls explain how D1 and D2, which are connected in an anti-parallel configuration, could minimize T1 saturation at high currents?



As I said before the core of the transformer can only hold a finite amount of energy, the reason the diodes are connected in this way is because of that.

The dot on the secondary coil indicates the direction the coil is wound onto the core.
You can think of this like the polarity of the coil, like the polarity of a battery.

Then when you consider the “push pull” drive of the primary you know that the energy stored in the core is first being stored in on direction then in the other.

With the diodes in this configuration one secondary will draw energy when it is stored (in the transformer core) in one direction (say from when the drive is pushing) then the other secondary will draw energy from the core when the drive is pulling.

Taking energy out of the core in this way will help stop it from saturating “filling up”


Hope that helped, Bob
 

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