EEVblog #504 - UPS Tutorial & Teardown

[xDR1TeK]

So, is that why it is called "inverter" because it uses the logical inverter transistor configuration (PMOS at top and NMOS at bottom)?
Well, the logical inverter is not a good circuit design for sine wave generation. If you say PWM generated sine wave, then the noise from this thing is awful.
What are the advantages of the inverter over the class AB amplifier circuit?
I have designed many of these full H-bridge class AB amplifiers, they are nice, powerful but very expensive.
If this should be discussed elsewhere, please direct me.

[theoldwizard1]

You kept mentioning P-channel MOSFETS. It seems as if it would make sense to use N-channel for the high side too with this kind of power. Did they really use P-channel? It seems the schematic you linked uses all N-channel MOSFETS.

That's correct. With a high side driver, it is correct to use N Channel devices. P Channel suffer from too many drawbacks to be a good fit for this function.

In other APC UPS teardowns this is clearly shown and adding additional N-Channel MOSFETs is the first step in increasing the power output like Dave said.

As others have said, almost all of the APC SmartPower UPS use a a similar design.  An important note is this mode is an extended run model (XL suffix) which has 1) the second Anderson plug for an external battery bank and 2) additional cooling (fans).

The second item is extremely important as Knurlgnar24 found out in his video  If you are interersted in UPS and modding them to use as straight inverters, check out Knurlgnar24 YouTube Channel

[theoldwizard1]

I forgot to add, this "style" of sine wave inverter ("low" voltage, high current, 50/60 Hz sine wave fed into a "iron" core transformer) is not the most efficient design.  There are significant losses in the transformer (note the 2 fans for the transformers).  This design probably yields about 80% overall efficiency. although this design works extremely well for UPS-inverters because of the way charging is done.

Typically, high efficiency inverter have 2 stages, a DC-DC boost stage and a sine wave generation stage.

The DC-DC boost converter is typically done with a SWPS push-pull design.  There are several different ICs that handle this.  MOSFET drive a ferrite core transformer.  After rectification and filtering the resultant DC voltage must be greater than the peak-to-peak desired output voltage (V_pp = SQRT(2) * V_RMS).  Other low voltage circuitry generates a "pure" sine wave which is sent into a Class D "amplifier" which switches (PWM) the high DC voltage.  a little filtering, some feedback and with careful design you can achieve 90-95% efficiency !

[tomar1]

good afternoon, someone would have the link available for this failure (I have one of the popular Back-UPS RS 500VA, which was sold with a design fault in the battery charging circuit)