Pushing OnSemi transistors to the limit

How hard can you REALLY push a single pair of OnSemi 4281/4302?

They are 230W spec, can you drive over 200W speaker at 4 or 2 ohms with a single pair for a short time, and have them live? How high could you run the rails? I was thinking of trying a pair on +/- 30 - 40 V at 2 ohms.

Like if you wanted to use them in a car amp, and use as few pairs as possible, what can you get away with?
 

wg_ski

Member
2007-10-10 5:21 pm
EWorkshop1708 said:
How hard can you REALLY push a single pair of OnSemi 4281/4302?

They are 230W spec, can you drive over 200W speaker at 4 or 2 ohms with a single pair for a short time, and have them live? How high could you run the rails? I was thinking of trying a pair on +/- 30 - 40 V at 2 ohms.

Like if you wanted to use them in a car amp, and use as few pairs as possible, what can you get away with?



I had a homebrew PA amp miswired for *over a year* before the distortion at 2 ohms got annoying enough to look into it. It was a darlington triple, with multiple MJ15024 outputs with the same driver, and a high-beta Jap predriver. +/- 70 volt rails. On one channel the output bank was shorted on the PCB base to emitter output rail. It was running off the driver! Not only that, the current limiting was effectively disabled. Lightly loaded, it sounded fine. Heavily loaded there was noticeable distortion, and I always wondered why the bias pot didn't like the same setting on both channels. A single 15024 was taking 2 ohm loads at +/-70v rails! It has since been fixed, but that does give some insight into how rugged these devices are. How long it would have taken it I don't know, but s/b is usually a quick death if it happens and by all rights should have happened the first time I hooked 4 speakers to each channel.
 
Wow! That's impressive. The 4281 seems to be somewhat similar to MJ15024 in power handling.

I'm not looking for HT or PA reliability, just reliable enough to last without too much abuse if kept cool. I'm using a large, thick piece of aluminum for heat sink. Also, the least paralleling I do, the more transistors can be used for other amp channels.

I sure won't be running 70 Volt rails in a car amp, but if a single pair at lower rails will push 2 ohms, then that's great news to save space and still have a decent amp. I've seen smaller transistors used with 100-120W RMS stages before, so I figure that the BIG TO264 transistors can probably handle it.

If I do parallel.....
Also, if I closely match transistors by voltage drop and Hfe, is it necessary to use emitter resistors when paralleling?
 

wg_ski

Member
2007-10-10 5:21 pm
Don't sweat it at low voltages. With 40V rails you're really only limited by average junction temperature. Provide plenty of cooling and don't skimp on the driver unless you want to run out of HFE and get distortion. A sinlge unit would hold - they push way beyond rated SOA for brief instances on most store-bought amps these days.

I wouldn't parallel without emitter resistors. You only need a tenth of an ohm if you bother to match them at all and if they're physically mounted within an inch of each other on the same heat sink.
 
using one pair of MJL4281, 200W into 2ohms 60degree phase angle is only just inside the 100mS SOAR at 25degC.
This was modeled at +-37.1Vdc, +-75mF of smoothing, Re=0r1

1r0 resistive just does not work at all.

Increasing Tc to 32degC brings the SOAR down to coincide with both the max current limit and max power limit for 100mS.

200W into 4ohms 60 degree phase angle is outside the 100mS Tc=25degC SOAR for the secondary breakdown voltages (60Vce @ ~7.3Apk) using +-48.9Vdc
 

wg_ski

Member
2007-10-10 5:21 pm
AndrewT said:
using one pair of MJL4281, 200W into 2ohms 60degree phase angle is only just inside the 100mS SOAR at 25degC.
This was modeled at +-37.1Vdc, +-75mF of smoothing, Re=0r1

If you analyze a known good amp, like the PLX3402, the SOA of it's 4 output devices per bank are exceeded by a *huge* amount. Peak power dissipation of 612 watts (per device, half way up the upper rail, 2 ohm resistive load). You shouldn't be able to operate it at all at 2 ohms at 20 Hz, but people do and it don't blow up until you start running it into thermal limit a few times.

Peak power dissipations of 200 watts (resistive) with a bit more reactive are no big deal for a 200W device. Those old Marantz boat anchors used to run 4 ohms off 40V rails (and some use to push lower) using old-school 100 watt Jap devices. And they held just fine.
 
I have no problem with advice offered so far, I thought I'd add some before you get far in the design phase. Running low impedance loads is a throw-back to the old days of car audio when only 12V was available and switching power supplies where for the rich. I don't understand why modern setups continue to push impedances lower, except of course in competition where amp ratings are fixed and it's a way to "cheat".

If you are building your own amp maybe you could design for higher voltage rails and use higher impedance speakers? A low current/higher voltage design is typically more efficient and easier on the output stage. Distortion is also improved at lower output currents. Of course this is all useless info if your low impedance system is already installed and paid for. High impedance like used in home audio is the way I'm going with my current build. I'm using 6-8ohm speakers and my amp will have no trouble with them. In my opinion even 4 ohm speakers are dinosaurs.;)
 

wg_ski

Member
2007-10-10 5:21 pm
Kevin_Murray said:
If you are building your own amp maybe you could design for higher voltage rails and use higher impedance speakers? A low current/higher voltage design is typically more efficient and easier on the output stage.

Yep. You can go to a quad 40V rail and go class H :). Then two pairs will do it at 4 ohms, you don't need to bridge, and it runs a LOT cooler.

I've been toying with the idea of cloning a Behringer EP2500 circuit for car audio. Rewind the existing trafo for a 12V primary, run it at 400 Hz old school inverter style and crank up 650 watts per channel with low heat.
 
wg_ski said:


Yep. You can go to a quad 40V rail and go class H :). Then two pairs will do it at 4 ohms, you don't need to bridge, and it runs a LOT cooler.

I've been toying with the idea of cloning a Behringer EP2500 circuit for car audio. Rewind the existing trafo for a 12V primary, run it at 400 Hz old school inverter style and crank up 650 watts per channel with low heat.


Now that's a project!:devilr:
 
Kevin_Murray said:
Running low impedance loads is a throw-back to the old days of car audio when only 12V was available and switching power supplies where for the rich.......................... In my opinion even 4 ohm speakers are dinosaurs.;)

WTF??? :rolleyes:

Since I can remember, for quite some time, most common car amps have 2 ohm channels so you have the option to bridge into a 4 ohm load!!!!!

Also, you may want the option to run 2 4 ohms speakers on each channel = 2 ohms/channel!

It has NOTHING to do with 12V.

16 or 8 ohms is more dinosaur because you have to use a high voltage mains powered amp and high voltage to get power, instead of having a heftier amp channel capable of pushing out some decent current into your speakers. Put a 4 ohm speaker on a wimpy 8 ohm channel, and you get distortion. Most lower ohm channels don't suffer so much when overloaded.
 
EWorkshop1708 said:


WTF??? :rolleyes:
<snip>
My apologies if my post came across as insulting. I'm not aware of "low ohm channels" though I am very familiar with electronics theory. I've also been around car audio for longer than I care to disclose. :ashamed:
What I meant was that high voltage/low current setups are more efficient than the opposite. The reason car audio has traditionally been low impedance is that there is not much voltage to begin with in a car. In a home or PA situation there is 120-240volts on tap and therefore designing an amp for higher impedances is an easier task. "Channel ohms" is not about machismo, rather it's about matching the impedance to the amp. An amp designed to operate into 8-16 ohms may have trouble with a 2 ohm load, but a "2kwatt" car audio amp would also struggle with PA speakers for example. Ohms law cannot be avoided.
I merely suggested changing the design philosophy of your system. I understand it wasn't well received but I meant no insult.:)
 
Have you folks here had good luck and power handling with the smaller 0281 and similar variants in the TO247 package?

I've killed a 6V lead-acid battery with a MJL4302, by pulling too much current, and the transistor lived! :bigeyes: Also they seem bulletproof in about anything I use them for. I use the PNP for battery chargers and the NPN for voltage regulators and fan speed controllers. They run cool and reliable and have lots of gain compared to most larger transistors.

Because they seem so tough, I'm tempted to push high power into speakers with a single pair and see how they do.

I remember when I was younger, I worked on an old Fisher amp using Toshiba Transistors 3281/1302 and it had 1 pair per channel @ 150W RMS per channel @ 8 ohms. Those are 150W transistors too, so they were being pushed to the limit. The On-Semi transistors seem much more rugged than the Toshiba.
 
One thing I forgot to mention is if you incorporate source (emitter) resistors into your amp you can then run a VA current limiting scheme. Even if you only use a single output pair per amp. This will protect your output transistors no matter what the load impedance.
I'm not experienced with the ON Semi devices but I have an amp design which uses the IRF 9240/240 FETs to great success. They are extremely robust and I used current limiting to boot.
 
Kevin_Murray said:
I'm not experienced with the ON Semi devices but I have an amp design which uses the IRF 9240/240 FETs to great success. They are extremely robust and I used current limiting to boot.

That does not count as MOSFETs do not have secondary breakdown limitations, which BJTs do, and which in fact prevent full power dissipation at higher C-E voltages.