During the recent educational event about equipment measurements by John Curl (Curl YouTube Event), one of the questions asked of John had to do with the use of Gallium Nitride (GaN) fets in amplifiers. Richard Schram of Parasound provides some additional information regarding the availability and use of GaNfets.
The following communication is from Paul Meister who is ICEpower's liaison with established audio companies in North America.
“I’ve copied the current state of the newsletter we are going to put out in the next month or so from our CTO of amplification Patrik, who you know. I think the short answer is we are not wowed by the current state of the output devices for amplification and have concerns about reliable thermal protection/monitoring, which is key for robustness that many of the applications we sell into require, while also not necessarily seeing audio performance gains compared to our latest topologies. Below is obviously a bit of “marketing speak” but is a true reflection of our current thoughts.”
“We all need to work to lower energy consumption, product weight and material usage in order to reduce our environmental footprint. Most of the amplifiers that are sold every year will be used at a very low output power during the majority of the life time when used to play background music and therefore the idle consumption is very important. In a class D amplifier, the idle consumption is usually very low because the ripple current in the output filter inductor commutes the PWM node from rail to rail automatically with basically no losses. The dead time is usually set so that the opposite switch turns on just when the PWM node touches the opposite rail. This dead time has two negative impacts. First, it creates a non-linearity due to the fact that this dead time varies with load. When an audio current is summed with the ripple current the PWM node will start to transition faster in one direction, due to the larger current in the inductor, and slower in the other as the transition will be delayed until the opposite switch is turned on. But it also gives a longer total propagation delay in the system which gives some limitations to what you can do to improve the audio performance. So, a lower output capacitance for a given ON-resistance is beneficial in order to speed up the self-commutation. At large output currents there is also a problem with the reverse recovery time of the body diode. Since the inductor current will freewheel through a body diode and the opposite FET will have to discharge this body diode while having full voltage across it, it leads to significant losses at high output powers.
GaN-devices offer some very interesting features in order to deal with these challenges. The low ON-resistance that can be achieved with a very small die size (low capacitance and tighter loops) in combination with no reverse recovery allows switching at a higher frequency using smaller magnetic devices and still achieving higher efficiency. The benefit of going to a higher fsw in a globally modulated self-oscillating class d amplifier lies in the fact that fsw will not drop down too close to the audio band (even during large output signals) hence allowing the audio band loop to have a higher gain without risking low frequency oscillation. There are however a few problems with the available GaN-devices as they usually are designed to be able to run at their full capability forever like in a power supply. Audio amplification is quite a different application as full power is only delivered short periods and even at full output power the average power is quite low. With most music types the average power is between 1/8th to 1/4th of max RMS power with some very bass heavy tunes excluded. This means that a smaller power device can usually be chosen which benefits performance, size, cost, EMI and idle losses/efficiency. But since the input signal cannot be fully controlled and shorting of speaker cables is not uncommon, the power devices still have to be able to survive a much higher power/current output than what music normally requires. The most critical situation is when the output is shorted with long speaker cables that form a load of a few hundred mohms. The amplifier will still oscillate but at a very high fsw and the output current will increase quickly once input signal is applied. It is not uncommon to see the die temp rise by 15-20 degrees per second which makes it difficult for the temp protection to keep up. The smaller thermal mass the power devices has and the higher the thermal resistance is from the PCB/temp sensor to the power device die, the higher the die temp will reach before protection kicks in. So, the connection from the GaN-device to an area increaser like a heat sink or the bare PCB with extra thick copper will be very important. As it is now, ICEpower has only used GaN for very special applications like a +/110V supplied current amplifier used for driving a capacitive speaker. But we have been testing them in lower voltage amplifiers used for 4-8 ohm speakers. As soon as we have something to show, I will make sure you will see it.
Regards,
Paul”
The following is from Richard Schram:
Paul Meister is ICEpower's liaison with established audio companies in North America. Like most suppliers, ICEpower has minimum order quantities. Companies that are too small to buy directly from ICEpower can only buy from distributors and do not get support from the factory.
A few months ago, ICEpower instructed Parasound and others instructing to submit our purchase requirements for products they will ship to us in 18 months. Illustrates supply chain "challenges."
The author of the GANfet newsletter is Patrik Bostrom, the Swedish chief engineer of a class D manufacturer in Sweden. We used these for Parasound class D products because they were superior to other class D manufacturers.
OEM project
We also sold Patrik's Class D amplifiers to Thiel for one of their "Smart Subs."
Jim Thiel asked Parasound to design and manufacture the input/eq board.
One of many Parasound OEM products we designed and built for audio companies. Most recently the Wilson Audio ActivAXO.