2 basic question about half bridge smps.

sbdada09

New member
Hi everyone, please do me a favor.
1. So far I found on the internet, these are the most common configuration of half bridge.



I want to know what are the advantage (or disadvantage) of each configuration or which one I should use in which situation.

2. In the case of audio smps, I found most of the time people use separate winding for 12V/15V or other output along with the main output winding, while in general purpose smps, they use tapping instead of separate winding. What is the advantage (or disadvantage) of separate winding over tapping?

Can you please give me some basic idea about these?
Thanks in advance :)
Regards,
Sukdeb.
 

Silvio

Member
@sbdada09

Please note that when using 2 bulk capacitors in the input the capacitance across the 320v is half. As a rule of thumb we use 1uf per watt of power across the 320v rail.

Fig 1 shows a basic arrangement with 2 divider capacitors but the DC blocking capacitor is omitted
Advantage Using less parts making the pcb more economical
Disadvantage 1 If in the event of failure of one of the mosfets the full charge of the capacitor will drain with a very high current.
Disadvantage 2 The value of capacitance across the 320vdc rail is half

Fig 2 shows the classical way of making a half bridge with 2 divider capacitors and a DC blocking capacitor to eliminate the danger from the previous arrangement of fig 1.
Disadvantage 1 the DC blocking capacitor takes a lot of beating and has to be of good quality such as film etc. The voltage rating must be higher than that of the half bridge itself.
Disadvantage 2 the rail capacitance is half.

Fig 3 Shows another classical arrangement but this time using only one bulk capacitor and only one divider capacitor. This is the most economical way of producing a low cost pcb and also space saving
Advantage The capacitor across the rail holds the full capacitance
Disadvantage The DC blocking capacitor takes a lot of beating and has to be of a very good quality as all the current has to pass through it.

Fig 4 This shows another arrangement this time using one bulk capacitor and 2 voltage dividing capacitors (C2 and C3) C4 is usually used in resonant applications where the capacitor resonates with the leakage inductance of the transformer.
Advantage 1 The bulk capacitor holds its full capacitance.
Advantage 2 the divider caps C2 and C3 will work at 50% duty and the capacitance value in them reflects to be double.

Fig 5 is the same as above regarding advantages this time the resonant capacitor is omitted. This configuration is uses mainly in hard switched and high power applications.

Regarding windings.
Normally in high power applications due to the fact that the high current secondary windings being made with multiple strands it would not be so convenient to introduce tappings, however separate windings are chosen so that a single or bifilar winding can be easily done. Auxiliary winding are normally used to supply fans and pre-amps.
In smaller smps then due to the winding wire is made of a single strand then it will be more convenient to use tappings from the same windings.

Regards Silvio
 
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sbdada09

New member
Thank you Mr. Silvio for your valuable information, these will help me a lot.
I'm little bit confused though, sorry for that. As you said, C4 in figure 4 resonates with the leakage inductance. Same configuration also used in figure 2 and 3 (C3). Doesn't it also convert the converter into resonant mode?
 

Silvio

Member
Yes it can resonate but it depend on the value and the switching frequency. However in the cases shown the capacitor is there to block DC
 
Sorry for digging up this thread, but I've never seen the Fig. 3 arrangement in an SMPS. Looks like a class A amplifier that has a DC blocking capacitor. I'm thinking that if this cap fails, the core would saturate from the DC voltage and blow the transistors. Has anyone tried it? Would a typical red film capacitor work?
 

Silvio

Member
Sorry for digging up this thread, but I've never seen the Fig. 3 arrangement in an SMPS. Looks like a class A amplifier that has a DC blocking capacitor. I'm thinking that if this cap fails, the core would saturate from the DC voltage and blow the transistors. Has anyone tried it? Would a typical red film capacitor work?
We usually use a CBB capacitor (C3)usually red sometimes blue etc. with a larger voltage than needed to make sure that it does not fail. This capacitor takes a lot of beating and all the load has to pass through it.
This arrangement is also used in smps. It covers both tasks to divide the voltage and also blocks DC. Economy at its best. I myself having a source of 450v electrolytic capacitors I usually use the configuration in figure 5. The stress in the divider capacitors is divided and the capacitance value is doubled. Thus dividing by 2x 1uF caps I will have a value of 2 uF. Each capacitor will work at 50% duty cycle. They also serve to block the DC voltage.
 

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