AN-D11

Supertex inc.
AN-D11
Application Note
±500 Volt Protection Circuit
Introduction
A ±500V protection circuit for low voltage high impedance
measuring instruments is shown in Figure 1. The protection
is accomplished by limiting the amount of current going into
the measuring instrument. The circuit will protect against destructive high voltages inadvertently connected to the probes
(VMEAS) of up to 500VDC of positive and negative polarity.
LND150N8
Probe
+
VMEAS
Probe
Q1
R1
Q2
V1
V2
Measuring
Instrument
Figure 1
Circuit Description
The circuit consists of two transistors, Q1 and Q2, and one
resistor, R. Both Q1 and Q2 are Supertex LND150N8, 500V
N-channel depletion mode MOSFETs with gate-to-source
ESD protection in a SOT-89 surface mount package. Q1
and Q2 are configured back-to-back as two constant current
sources with a nominal value of 1.0mA. Resistor R sets the
current limiting value. Figure 2 is a typical low voltage high
impedance measurement instrument. Figure 3 is a simplified
equivalent circuit showing the protection scheme.
+
V1
V2
1/4 LM324
V0 = 2(V2 - V1)
Figure 2
Under the fault condition, the absolute value of VMEAS is
greater than the supply voltage, Q1 limits the current to 1.0mA
against large positive voltages and Q2 limits the current to 1.0mA against large negative voltages across VMEAS.
For example, if VMEAS is connected to ±500V, Q1 and Q2 will
limit the input current to ±1.0mA causing the input voltage to
the measurement instrument to clamp to 1.3V above its supply voltage (when R = 600Ω) and 0.7V below ground.
Typically the measuring instrument has ESD protection diodes connected from both probes to its power supply and
ground. The ESD protection diodes can usually handle
1.0mA continuously. In case there are no ESD diodes provided, external diodes D1, D2, D3, and D4 can be added.
VSUP
VSUP
+
100k
VSUP
_
Under normal operation, the absolute value of VMEAS is less
than the supply voltage of the circuit. Q1 and Q2 will be fully
on with a maximum guaranteed RDS of 1.0kΩ. Since the instrument’s input impedance is typically very high, say above
10MΩ, the additional 2.0kΩ series resistance from Q1 and
Q2 will not affect measurement accuracy.
LND150N8
1k
100k
100k
VSUP
100k
+
1/4 LM324
D1 V
1
R
VSUP
1.0mA
D2
-
D3
R
1.0mA
+
VMEAS
VSUP
VSUP
Probe
V2
Probe
D4
Internal ESD Protection
Figure 3
Doc.# DSAN-AN-D11
A040213
Supertex inc.
www.supertex.com
AN-D11
Calculation for Resistor Value
PDISS = 600mW = (constant current value)(max. input
voltage)
For a current limiting value of ±1.0mA, R can be approximated by the following equation:
Figures 4A and 4B are pictures of current due to VMEAS vs.
VMEAS voltage of the actual circuit. R1 was chosen to be
1.0kΩ.
VGS(OFF)
R1 =
• ( √ ID / IDSS - 1)
I
D
where,
ID = desired constant current value,
VGS(OFF) = pinch-off voltage, and
IDSS = saturation current at VGS = 0V.
Conclusion
The high voltage protection circuit is ideal for both bench
measurement and handheld measurement instruments. It
is simple, reliable and cost effective. It eliminates the possibility of input damage to very sensitive and expensive high
impedance devices within the measurement instrument.
VGS(OFF) and IDSS are device characteristics and will vary
from lot to lot. Actual constant current values are not critical
as long as the power dissipation of the LND1 is less than
600mW.
Figure 4A
Figure 4B
Supertex inc. does not recommend the use of its products in life support applications, and will not knowingly sell them for use in such applications unless it receives
an adequate “product liability indemnification insurance agreement.” Supertex inc. does not assume responsibility for use of devices described, and limits its liability
to the replacement of the devices determined defective due to workmanship. No responsibility is assumed for possible omissions and inaccuracies. Circuitry and
specifications are subject to change without notice. For the latest product specifications refer to the Supertex inc. (website: http//www.supertex.com)
Supertex inc.
©2013 Supertex inc. All rights reserved. Unauthorized use or reproduction is prohibited.
Doc.# DSAN-AN-D11
A040213
2
1235 Bordeaux Drive, Sunnyvale, CA 94089
Tel: 408-222-8888
www.supertex.com
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