A DC Fault Protection Circuit for Audio Amplifiers

Copyright 2001 R.G. Keen. All rights reserved.
No permission for local copies or serving from hosts other than
http://www.geofex.com.

The standard Class AB audio power amplifier allows for direct coupling of the
output of the amplifier to speakers. This is very good in that no capacitors or
transformers get in the way of the sound quality coming out. The speakers are
connected directly (more or less!) to the amplifying devices. This has the
unfortunate side effect that if an output device fails, this usually causes the
raw DC power supply to be connected to the speakers. Most speakers burn out or
are mechanically damaged by this very quickly.

Protection circuits have been designed to prevent this kind of damage, and
come in many flavors. They range from the simple - such as an added fuse in
series with the amplifier output - to the complex, with all sorts of
monitoring. The current favorite is a series relay circuit at the output of the
amplifier, driven by some sort of DC detection circuit.

Relays have their own set of problems, though, not the least of which is
reliability. Relay contacts corrode, arc and stick over time. Worse, each time
they are tested could well be the last time they operate, so there are some
built in problems. Fuses are also problematical, as they must be excruciatingly
well sized to offer good protection, and when that is done, the fuse resistance
is modulated by the heat dissipated in the fuse near maximum power, so there
are audible side effects.

Some recent advances in power MOSFETs make them attractive for replacing or
sidestepping relays and fuses on the output of the power amp. Power MOSFETs
make good switches, changing from a totally nonconductive state to a
fractional-ohm resistor in nanoseconds with the proper drive signal. Unlike
bipolar transistors, there is no offset voltage or rectification associated with
a MOSFET's on-state conduction. As long as the current through the MOSFET is low
enough to not cause a significant voltage drop across the MOSFET during current
peaks, the MOSFET itself will not be audible.

There are some difficulties with driving MOSFETs as AC relays, though. There
is an inherent substrate diode in all MOSFETs that looks to the outside world
like a diode connected across the source-to-drain. This diode conducts freely
in the reverse direction. This can actually help protect the MOSFET in some
situations, but it means that a single MOSFET can't block current in both
directions. However, putting two MOSFETs source-to-source can. The substrate
diodes then are alternately reverse biased on either signal polarity. Suitable
enhancement of gate-to-source voltage then gives us a bidirectional AC
switch.

The sources are always riding on the instantaneous signal level, though, and
both gates must be made about 10-12V more positive than that moving signal
level. This is very difficult to do for most floating drive circuits.

International Rectifier has released several photovoltaic MOSFET
drivers. These units comprise an LED to produce light, and several photovoltaic
diodes in series. When the LED shines on the photodiodes, they produce a voltage
on the output pins of the unit.

In the direct replacement for an output relay, two N-channel power MOSFETs
are connected source-to-source and gate-to-gate. The drain terminals are used
as the input and output of a switch. When the gate-source terminals of the two
MOSFETs have 0V between them, the devices are off, and no current can flow, up
to the breakdown voltage of the MOSFETs. When the gates of the MOSFETs are
several volts positive with respect to the sources, both MOSFETs are enhanced,
and current can flow. Modern MOSFETs can have a resistance from drain to source
in the milliohm region.

The photovoltaic isolator makes for a simple, easy, way to turn the two units
on or off because the voltage-source side of the isolator rides along with the
source terminals while the LED side may be tied to any level necessary for
operation. To turn the "relay" on, just force current through the LED. To turn
it off, just interrupt the LED's current.

There are people who will instinctively distrust any active solid state
device in series with an audio signal. For those people, there are two possible
alternatives: a relay to "short" the two power MOSFETs of the circuit above and
a second MOSFET protection circuit that interrupts the DC power supply, not the
speaker output. The first idea is simple - relay ills are often caused by the
wear and tear of interrupting an ongoing current. If we make the series MOSFETs
conduct just slightly before the relay contacts close and hold for a tiny slice
of time afterwards, then the MOSFETs endure all the voltage and current stress
of actually closing and opening the connection. The relay serves only to make
for a low-resistance path around any possible perturbation of the sound by the
MOSFETs.

The second version inserts the N-channel MOSFETs into the DC power
lines going from the power supply to the amplifier itself. Each MOSFET gets its
own photovoltaic isolator. When current flows through the LEDs in the isolators,
the MOSFETs are turned on, and current flows to the amplifier.

This version of protection is most easily thought of as a very fast relay
that can turn an amplifier's power supply off in microseconds. While the
amplifier circuit might have bypass capacitors that will still drain through the
speakers, this will be a far smaller amount of energy than the main power supply
with its large filter capacitors would feed the speakers, and so they ought to
survive in good order.

In this application, the reverse substrate diode does not matter because the
voltage is always in the correct direction to reverse bias it.

This version uses two of the photovoltaic isolators because the DC voltages
on the gate/source of the two MOSFETs are widely separated.

Another variation of this approach is to use a P-channel MOSFET in the +Vcc
line and an N-channel MOSFET in the -Vcc line, and drive both gates towards
ground with the protection circuitry for "on" and release the drive for "off".
This approach works, and suffers only in that P-channel MOSFET devices usually
have a more limited voltage and current range than N-channel devices. They are
likely to be more expensive as well.

However, if you're willing to trade more expensive power MOSFETs and more
extensive drive circuitry off against the cost and availability of the
photovoltaic isolators, This circuit can work well.

The drive circuit is worth some discussion. To turn a power MOSFET fully on,
you typically need 10 to 12V of enhancement on the gate-source. However, if you
go much above this, the gate oxide can be punctured. Just to be sure that the
gate never gets outside the safe area, each MOSFET gets a 12V zener to clamp
the gate voltage to no more than 12V. The resistor parallel to the zener diode
ensures that the gate is pulled to the source voltage if there is no other
drive on it. 10K to 100K works well there.

The NPN/PNP transistor pair in the center have their collectors tied to
pull-down resistors to the MOSFET gates. These resistors are sized so that the
Vcc voltage minus 12V of gate enhancement voltage produces a reasonable
current, well within the zener's current and voltage rating. This needs to be
sized appropriately for each different Vcc level.

The PNP transistor's base is tied to ground through a resistor, and the NPN
transistor's base is the control point. Both emitters are tied together so that
whatever current the NPN transistor conducts also goes through the PNP. When
the NPN transistor's base is pulled above two diode drops, one base-emitter for
each transistor, the bases of both transistors start conducting. The two base
resistors are a safety net that ensures that the bases won't be damaged by
excessive base current. Above about 1.4V, the control pin saturates the NPN/PNP
control pair. The same control signal turns on both the NPN (in common emitter
mode) and the PNP (in common base mode), and they both pull their respective
MOSFET gates into full enhancement, and therefore full conduction.

The MOSFETs must be rated for the full voltages and currents that the power
supply can be expected to produce. For an amp with +/- 50V Vcc rails, get a
MOSFET with more than 100V, preferably more than 120V Vdsmax. Also, since
current rating is cheap in MOSFETs, get devices that ensure that there is
enough current handling capability to not "choke" the output stage of the amp.
If necessary, you may freely parallel MOSFETs to get more current capability.
Since these devices do not handle any audio (other than that audio that is
passed through the power supply) and since they are always either fully
saturated or fully off, the audio performance of the MOSFETs is really not an
issue. As long as the on resistance of the MOSFET is comparable to the source
resistance of the power supply, this should be pretty transparent, especially
for modern power amps with large power supply rejection ratios.

MOSFET current rating is an issue as well. Power MOSFETs come in some
surprisingly high current ratings for the dollars you spend, and they parallel
beautifully. If you have a really high power amplifier, with maybe +/- 100V
supplies, you'll need 200V or more rated MOSFETs. You can simply parallel as
many same-type MOSFETs as you need to get a high enough current rating. They
will safely share the current they carry. An interesting point that sets
MOSFETs apart from bipolar transistors as power switches is that paralleling
additional devices does NOT require more drive current or power. All the
parallel MOSFETs you use will saturate as fully as a single one (within the
limits of their individual transconductance rating, of course). All that changes
is that the total gate-source capacitance that the voltage drive to the gate
must charge and discharge goes up as you parallel more gate-source capacitors.
This means that the transition between off to on and on to off again slow down,
not that the devices are less fully on or off. Since the transition is already
so fast that fuses look like glacial motion beside it, this isn't much of an
issue. It's likely that little or no heatsinking is needed on the MOSFETs
because they dissipate large amounts of heat only while they are actually
switching - which they don't do very often.. Simple engineering caution dictates
that you don't just assume that they dissipate no heat, but the amount is minor
compared to the output devices. The sink's thermal capacitance (that is, how
long it takes to heat up the sink itself) may be more important than the thermal
resistance of the sink because the MOSFETs normally switch only twice per
session  - once on, and once off.

Devices like the IRF540 n-channel and the IRF9540 p-channel are good types to
do this with. Depending on the maker, the IRF540 is rated at up to 27 amperes
and the IRF9540 is rated at 19A. Two 9540's in the positive rail and two 540's
in the negative rail  makes for a very efficient and high-current-rated
protection switch.

As to the circuitry that senses fault conditions and turns the photovoltaic
isolator LEDs on and off, there are many ways to accomplish that, and in fact,
any circuit that is a DC protector that drives a relay can be rearranged to
drive the photovoltaic isolator LEDs  as well. These circuits usually
encompass a single-pole RC low pass filter with its rolloff arranged so that the
highest-level and lowest-frequency bass notes will not cause the circuit to
trip, but a DC level will cause a trip within a few milliseconds. For any of the
circuits shown here, the detector circuit merely has to energize the LED(s) when
there is no fault, and drop that current when a fault occurs.

It's worth noting that you're not limited to DC overcurrent sense as a trip
condition. Other good conditions that can be used to activate the protection
circuit are overheated heat sinks or power transformer, sensing RF
(oscillation) on the output, and so on. With appropriate logic (CMOS works very
well here) these can help keep your amp alive.