Class D amp standby power not very low - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: June 26, 2020, 02:12:41 am
content: Been playing with a couple of these Class D amplifier modules. DC rails are +/- 67VDC. On a tone burst they will put out 225W RMS into 8 ohms. I just thought I’d measure the power consumption at standby. With two modules connected, the AC power from the mains is 31 watts! I think a lot of it is going into heating up the output HF filter inductor. After idling for 1/2 hour their temps get to 85 deg C in a 15 deg ambient. Not exactly the last word in efficiency! I think the switching frequency is about 375kHz so there might be a lot of skin effect losses in the inductor. Might be interesting to put on a bigger external inductor and see what happens.

author: Dave
author_rank: Super Contributor
timestamp: June 26, 2020, 03:18:14 pm
content: Try measuring the power consumption of the transformer with the secondary disconnected. You might be seeing a fair bit of core losses in there if the peak flux density is a bit on the high side.If you have the equipment to do it safely, observe the primary current waveform with a scope. The waveform will not be very sinusoidal if the transformer is close to saturation.

author: duak
author_rank: Super Contributor
timestamp: June 26, 2020, 04:42:33 pm
content: Having designed a PWM driver with a 100 kHz switching frequency I'll say that all the components show frequency related switching losses. FETs, diodes and inductors in particular. An air core inductor could be advantageous at higher frequencies but it may cost due to EMI, physical size and resistive losses.

author: tooki
author_rank: Super Contributor
timestamp: June 26, 2020, 06:58:24 pm
content: Quote from: Circlotron on June 26, 2020, 02:12:41 amTo an extent, the quiescent power consumption of a class D amp scales linearly with the power supply voltage. At the same time, the maximum power output of the amp is directly limited by the power supply voltage. Normally, this means choosing the balance of maximum output vs. efficiency at low output levels. Indeed, I’m working on an audio project at work involving precisely this tradeoff, and I thought to myself, “you’d actually want to vary the supply voltage depending on the power requirements”, and I thought about using the MCU, which will already control volume digitally, to control the voltage of the PSU. Right after this, I randomly came across some Texas Instruments reference designs for something even better: they call them “envelope tracking power supply” designs. They simply use some op-amps to prescale the audio signal into a feedback signal for a DC-DC converter, causing the supply voltage to closely track the audio signal. Apparently this is used extensively in some RF applications, but is practically unknown in audio. I found some research papers discussing it, with quite poor experimental results, but the TI designs from just a few years later have excellent performance according to their test results.Maybe one of TI’s designs could be adapted to your application.

author: mikerj
author_rank: Super Contributor
timestamp: June 26, 2020, 07:16:44 pm
content: Quote from: tooki on June 26, 2020, 06:58:24 pmClass G/H linear audio amplifiers use supply rail modulation (though in a very quantised manner for Class G) and have been around for quite a long time.

author: langwadt
author_rank: Super Contributor
timestamp: June 26, 2020, 08:09:12 pm
content: Quote from: tooki on June 26, 2020, 06:58:24 pmtracking the supply to the signal is definitely not unheard of in audio, it is called class-H or G with either continuously variable or stepped supplyCarver went even further with both multiple supplies and basically a triac dimmer on the primary side of the power transformer

author: TheMG
author_rank: Frequent Contributor
timestamp: June 27, 2020, 02:16:08 am
content: Probably more due to losses in the ferrite core. Might be a bit too small or not quite the best choice of ferrite material for the frequency.That being said, it's not that unusual for class D amps to produce some noticeable heat while idle. They are still considerably more efficient than a class AB design, just not necessarily at idle. You'll probably notice the inductor won't get much warmer when pushing the amplifier really hard, compared to idling.

author: Circlotron
author_rank: Super Contributor
timestamp: June 27, 2020, 03:05:54 am
content: Quote from: Dave on June 26, 2020, 03:18:14 pmThe bare transformer is pulling 13-14 watts. It's size is just under 300VA, so if you think the wattage figure is a bit high I'll put the scope on it and post a screen shot. I have a Hall effect current probe.

author: Circlotron
author_rank: Super Contributor
timestamp: June 27, 2020, 10:41:38 am
content: Transformer primary current = 196mA at no load.Primary current with and without load.