AV8 Power Supply Board
The 40W Resonant supply is designed to produce
extrem ely low le vels of con ducted and radiated noi se. 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 th e 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 s upplies.
! Digital supplies
The digital supplies are for use on non audio circuits e.g
microprocessor and similar circuitry, they are referenced to
DGND.
! Analogue supplies
The analogue supplies are for use on audio circuitry, they
are referenced to AGND.
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 out put curren t 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)
! Adjustab le supplies
o+40V (adjustable)
o+7V5 (adjustable))
These supplies are floating to allow them to be configured
as positive or negative supplies to suit the particular
display used.
The ou tput s are semi -regula ted b y a zener/em itt er follo wer
circu it. The Outp ut volta ges my be trim med by additi on 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 Synchronisatio n Input
The free running f r equency 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 leve l 50%du ty cycle sq uare wave.
External PSU Kill Signal
For applications which require a means of shutting down
th e PSU (e. g. i n t h e ev en t of a d etec t ed fa u lt sc ena ri o) t h en
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 d isa ble gat e dri ve sign als and the PSU ou tpu t volta ges
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
int end ed to p rotec t th e PSU an d it s loa d in the ev ent of th e
feedback signal failure.
In this design both the 3V3 and +5V are sensed. In the
even t of a fai lure i n e. g. t he 5 V rai l then the fe edb ack 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 even tualit y.
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.
Digital supply
rails Voltage tolerance Max cu rrent
+3V3 3% 1.5A
+5V 3% 1A
+12V 12.9V 5% 500mA
-12V 12.9V 5% 500mA
Analogue supply
rails Voltage tolerance Max cu rrent
+20.8V 5% 300mA
-20.8V 5% 300mA
+18V 2% 300mA
-18V 2% 300mA
Adjustable supply
rails Max current
+21V 30mA
5V 150mA