AP13
AP13: UltraVolt HVPS OutputCurrent Monitor
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UltraVolt's highvoltage power supplies (HVPSs) have an outputcurrent
monitor (Iout Monitor), which allows the total output current of the HV
multiplier to be monitored. Note, since the sense
circuit is in the return path to the transformer, the currentmonitor signal
has a sign opposite of
the sign of the HVPS. Therefore, a negative HVPS has a positive currentmonitor
voltage and a
positive HVPS has a negative currentmonitor voltage.
The current monitor is generated via an internal sense
circuit and brought to the output pin through
an isolation resistor. The signal can be integrated by connecting a small
capacitor between the Iout Monitor and the Signal Ground Return to form an R*C with the internal isolation
resistor. The
isolation resistor varies with the HVPS series as does the internal sensecircuit
impedance, which varies from model to model. The “A” Series
has a 15kΩ,
1% isolation resistor. The 60W, 125W,
and 250W“C” Series units have a 5kΩ, 1% isolation resistor.
The 20W and 30W“C” Series units
have no isolation resistor because pin 3 is used for grounding (to achieve
boost). The Iout Monitor
output impedance is therefore very low (varying from 22Ω to <1Ω depending
on the model).
The current monitor indicates all
current flowing from the HVPS HV multiplier. This total current is
comprised of the external HV load placed on the supply, the internal HV
resistor used to provide
a feedback signal to regulate the HVPS, and the internal HV resistor for
the Eout test point (if the
unit is so equipped). To develop a “true” outputcurrent monitor,
the leakage current in the HVPS internal resistor(s) has to be subtracted
from the current monitor. If the current monitor is fed to a
computer through an analogtodigital converter, this nulling can be achieved
in software by using
Ohm's Law. To calculate the internal leakage current, divide the output
voltage by the total of the
internal resistor(s) and subtract that current from the current measured
on the currentmonitor
pin. If the current monitor is used in an analog circuit, a simple analogsumming
circuit can be
created using the outputvoltage monitor as a correction current. The analogsumming
circuit is
easily implemented, since the polarity of the outputvoltage test point
is opposite that of the
outputcurrent monitor.
It is important to note, the internaldivider
leakage current is directly proportional to output voltage
and reduces linearly to zero as output voltage is reduced toward zero.
The leakage current
represents a higher percentage of the currentmonitor signal in higher
voltage models and a lower
percentage in lower voltage models. As an example, the 1/4A24P30 has an
output of up to 250VDC @ 120mA, and a 2.5MΩ internaldivider resistor
with a current of 100µA. The internal
current is <0.085% of the outputcurrentmonitor signal, not significant
enough in most applications
to null out. On the 20A12P4, the output is up to 20kV @ 200µA and
the internaldivider resistor
has a current of 40µA. The internal current is >16% of the outputcurrentmonitor
signal.
UltraVolt High Voltage Power Supply Total Internal HV
Divider Resistance
“AA” Series
Model 
4/20/30 Watt 
1/16AA 
560k 
1/8AA 
1.1 Meg 
1/4AA 
2 Meg 
1/2AA 
5.4 Meg 
1AA 
40 Meg 
2AA 
67 Meg 
4AA 
100 Meg 
6AA 
151 Meg 
“A” Series
Model 
4/15/20/30
Watt 
“AF”
Version 
1/8A 
1.25 Meg 
91.7K 
1/4A 
2.5 Meg 
2.0 Meg 
1/2A 
5.0 Meg 
3.3 Meg 
1A 
4W/50M, 20/30W 10 Meg 
8.3 / 5 Meg 
2A 
100 Meg 
50 Meg 
4A 
200 Meg 
66.6 Meg 
6A 
300 Meg 
75.0 Meg 
10A 
250 Meg 
 
15A 
375 Meg 
 
20A 
500 Meg 
 
25A 
625 Meg 
 
30A 
750 Meg 
 
35A 
2.22 Gig 
 
40A 
2.22 Gig 
 


“C” Series
Model 
20/30 Watt 
60/125/250Watt 
1/8C 
909k 
990k 
1/4C 
2 Meg 
2.43 Meg 
1/2C 
3.3 Meg 
4.76 Meg 
1C 
5.0 Meg 
9.09 Meg 
2C 
50.0 Meg 
50.0 Meg 
4C 
66.6 Meg 
66.6 Meg 
6C 
75.0 Meg 
75.0 Meg 
8C 
 
285.7 Meg 
10C 
 
333.3 Meg 
12C 
 
375.0 Meg 
15C 
 
500.0 Meg 
20C 
 
667.0 Meg 
25C 
 
770.0 Meg 
30C 
 
858.0 Meg 

Rev. U 07/10
*** END OF APPLICATION NOTE #13 ***