Arcam AV8 External Synchronisation Input, External PSU Kill Signal, Under-voltage lockout

AV8 Power Supply Board

The 40W Resonant supply is designed to produce extremely low levels of conducted and radiated noise. The supply is based around a discontinuous inductor current single ended flyback topology with active clamp. The active clamp eliminates the power losses normally associated with RCD type clamps and also eliminates the high voltage ringing waveform normally present on the primary inductance. In addition current is transferred to the output capacitors as a half wave sinusoid rather than a triangle wave which would normally be expected from a flyback supply. The sinusoidal current waveform into the output capacitors results in reduced output noise with attenuated high frequency response.

The PSU provides the following functionality

!Universal input voltage range 85V ac rms to 265V ac rms.

!Eight regulated supply rails.

The output voltages are provided as 3 groups of supplies.

!Digital supplies

The digital supplies are for use on non audio circuits e.g microprocessor and similar circuitry, they are referenced to DGND.

Digital	supply	Voltage tolerance		Max current
rails
+3V3		3%		1.5A
+5V		3%		1A
+12V		12.9V	5%	500mA
-12V		12.9V	5%	500mA

!Analogue supplies

The analogue supplies are for use on audio circuitry, they are referenced to AGND.

Analogue supply	Voltage tolerance	Max current
rails
+20.8V	5%	300mA
-20.8V	5%	300mA
+18V	2%	300mA
-18V	2%	300mA

Note. The rated supply currents for the analogue supplies are 300mA per rail. The +18V and –18V supplies are provided by 3 terminal regulators which derive their current from the +21V and –21V rails. The current ratings can thus be written.

Sum of output current from +21V and +18V = 0.3A max Sum of output current from -21V and -18V = 0.3A max

AGND and DGND are joined together at 1 place on the PSU (LK1)

!Adjustable supplies

o+40V (adjustable) o +7V5 (adjustable))

Adjustable supply	Max current
rails
+21V	30mA
5V	150mA

These supplies are floating to allow them to be configured as positive or negative supplies to suit the particular display used.

The outputs are semi-regulated by a zener/emitter follower circuit. The Output voltages my be trimmed by addition of resistors between the lines +40V_SET and +5V_SET.

Note. The +40V rail was originally to be set to +36V as marked on the drawings but it was found that the display was to bright and subsequently has been adjusted to +21V.

External Synchronisation Input

The free running frequency of the PSU is around 75KHz. For applications which require synchronisation to a system clock, the PSU may be entrained to an applied Sync signal. The Sync signal should be between 85KHz and 100KHz. The Sync signal is applied via line SYNC (SK8-1) and should be a TTL level 50%duty cycle square wave.

External PSU Kill Signal

For applications which require a means of shutting down the PSU (e.g. in the event of a detected fault scenario) then PSU latched shutdown may be initiated by pulling and holding line PSUKILL* to DGND. The PSU will enter a latched shutdown state after around 1second. The Latched shutdown forces all output supplies to 0V and can be defeated only by removing the mains input to the PSU for >15seconds.

Under-voltage lockout

The PSU is protected against the input voltage being too low (which would otherwise cause the input current to be excessive for a given output power). In the event of the input voltage being <75V ac rms then the PSU control IC will disable gate drive signals and the PSU output voltages will fall to zero.

Output Over-voltage Protection

The output voltages are monitored and in the event of the output voltages rising approx 15% above specified limits for >0.5 seconds then a latched shutdown event will be triggered. This mechanism is not intended to protect sensitive components from damaging transients but is intended to protect the PSU and its load in the event of the feedback signal failure.

In this design both the 3V3 and +5V are sensed. In the event of a failure in e.g. the 5V rail then the feedback loop will try to make the other rail voltage high enough to supply all of the feedback current which the other, now broken, supply was formerly contributing toward. This would normally force the 3V3 supply to more than twice its expected value (and all of the other supplies would be multiplied by the same factor). The provided mechanism prevents this eventuality.

Feedback loop failure protection

In the event of the feedback loop failing (eg. due to a broken feedback component) then the supply will enter latched shutdown after around 1 second.