Solid state switch has following capabilities:  (1) Hybrid or IC from compatible with direct mounting on solar array substrate; (2) Continuous signal is not required to hold it in either on or off state; (3) Separate signal lines for on and off control; (4) Electrically isolated from input signals; and (5) Current surges will not cause switch failure

Gooder, S. T.

Hoffman, A. C.

English

NASA Technical Reports Server

SWITCH POWER SUPPLY 
.
DDI
D2jfp 
LOW VOLTAGE . HIGH VOLTAGE
;0LAR ARRAY SEGMENT 
August 1974	 B74-10069 
NASA TECH BRIEF 
Lewis Research Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Self-Protecting Solid State Isolated Switch 
The Problem: 
One of the new concepts in space power systems 
involves the delivery of regulated power directly from a 
solar cell array to a load. This approach eliminates the 
power processer with its attendant losses and offers the 
potential for a lighter weight, more efficient and more 
reliable power system. Since the output voltage of a solar 
array varies with temperature, cell degradation and load 
current, some means of regulating the array output 
voltage must be employed. One method of providing this 
regulation is to short out blocks of solar cells with a 
switch. If the switch voltage drop is low, the resulting 
power dissipation is also very low since the shorted cells 
will be operating near their short circuit point. 
Some loads, such as ion thrusters, tend to arc 
periodically. This causes a short across the solar array 
supplying that load. During this condition, stored energy 
can result in large currents flowing through switches
which are in the "on" condition. Since the drive to these 
switches is limited, very high power dissipation could 
result if they remained in a conductive state. 
The Solution: 
A solid State switch having the following capabilities: 
1. Hybrid or IC form compatible with direct mounting 
on the solar array substrate; 
2. Self-latching such that a continuous signal is not 
required to hold it in either the on or off state; 
3. Positive switch control by having separate signal 
lines for on and off control rather than a single line with 
the first pulse turning the switch on and the second 
identical pulse turning the switch off; 
4. Electrically isolated from input signals so that 
supplies which float at high potentials (1 kV to 15 kV) 
can be regulated; and
(continued overleaf) 
This document was prepared under the sponsorship of the National
	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19740000069 2020-03-17T14:02:51+00:00Z
5. Self-protecting such that current surges which may 
occur during load transients (such as ion thruster arcs) will 
not cause switch failure. 
How It's Done: 
As shown in the figure, the switch is turned on by 
providing a current pulse to LED Dl. The light output 
from Dl activates transistor Qi allowing Q3 to turn on 
thereby providing base drive to Q4. Once Q4 reaches 
saturation, the path through D3 provides a latch to hold 
the switch on. 
To turn the switch off, a current pulse is provided to 
LED D2. The light output from D2 activates Q2 allowing 
Q5 to turn on. Q5 then shorts out the base drive to Q4 
turning the switch off. 
The optical path between the LED's Dl and D2 and 
their respective transistors Ql and Q2 provides the 
electrical isolation between the control signals and the rest 
of the circuit. Other than for diodes Dl and D2, the entire 
circuit including its power supply floats at the array 
segment potential. This can be as high as 15 kV for some 
anticipated loads. 
The voltage at which the power supply will operate is 
the sum of the emitter-base drop of Q3, the drop across 
D3 and the collector-emitter drop of Q4. By sizing the 
power supply voltage at the knee of the V-A characteristic 
to be higher than this voltage summation, the supply will 
operate in the constant current region. By this means, the 
power supply can be used to control the base drive 
current to Q4. During a flight mission, the current output 
of the switch power supply will degrade due to radiation 
damage. However, the current capability of the solar array 
being controlled will also degrade. This allows the ratio of 
base drive current to the collector current to remain the 
same over the mission lifetime. 
The switch power supply characteristic is used to 
provide protection for Q4 during transient conditions. As 
the current through Q4 increases, its V ce will also increase 
driving the operating voltage of the switch power supply 
up toward the knee of the curve. As the operating point 
moves around the knee, the base drive current to Q4 starts 
to decrease, further increasing Vce of Q4. Eventually the 
power supply is no longer capable of providing enough 
voltage to maintain current through the latch leg and Q3 
is turned off which in turn shuts off Q4. If the voltage of 
the switch power supply is sized properly, this turn off 
will start to occur before Q4 has pulled out of saturation. 
This prevents the situation froni occurring where Q4 
could operate for long time periods in a non-saturated 
condition. 
During a flight mission, the voltage of the switch power 
supply will degrade. The voltage drops VEB of Q3 and V D 
of D3 will also vary. This means that the potential 
difference between the operating region and the knee of 
the V-A characteristic will vary with time. Therefore, the 
conditions under which the switch will self protect will
also vary. In this particular application that is no problem 
as long as the switch will remain on carrying its maximum 
worst case current and turn off if Q4 attempts to pull out 
of saturation. However, if the degradation were quite 
severe, it might be necessary to regulate the switch power 
supply by placing a number of forward biased diodes 
across it. 
The diodes D4 and D5 are used to prevent false turn-on 
in the switch during solar array transients. If the particular 
array segment which the switch is across has a somewhat 
lower I sc
 than other segments, it will reverse polarity 
during a short across the entire array. This reverse polarity 
would turn on the switch through D3 if no other 
protection were used. 
The R and C across the emitter-to-base junction of Q3 
are used to put a slight delay in the turn on. This reduces 
the possibilities of triggering due to noise. 
Notes: 
1. The switch power supply is made up of a small number 
of solar cells connected in series. The current output of 
a solar cell is a function of its area. Since cells are 
available in a large range of sizes, the current output of 
the power supply can be varied over a wide range. The 
voltage output of a solar cell is a function of the cell 
resistivity. By using cells of different resistivities and 
varying the number of cells in series, the output voltage 
of the supply can also be varied over a wide range. The 
characteristics of this supply are used to provide a 
number of functions for the switching circuit. 
2. By grouping the solar cells in binary fashion, very fine 
regulation can be obtained with a few shorting 
switches. 
3. No additional documentation is available. Specific 
technical questions, however, may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B74-10069 
Patent Status: 
Inquiries concerning rights for the commercial use of 
thisinvention should be addressed to: 
NASA Patent Counsel 
Mail Stop 500-113 
Lewis Research Center 
- 21000 Brookpark Road 
Cleveland, Ohio 44135 
Source: A.C. Hoffman and S.T. Gooder

Lewis Research Center 
(LEW. 1 2268)
r
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B7410069	 Category 01


Self-protecting solid state isolated switch

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