AN-7517| Application Note
Practical Aspects of Using PowerMOS Transistors to Drive Inductive Loads
Application Note October 1999 AN-7517
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Many of the more recent applications of PowerMOS transistors, particularly low voltage devices, have been as solenoid drivers. In this type of application the device is simply used as a switch to turn the current through a solenoid, relay or other inductive load on and off (Figure 1). Since the dissipation is low, a very small or no heat sink will be required. This note will cover the application of the rating and characteristics of PowerMOS transistors to that type of application and illustrate the process of selecting a suitable transistor.
transistors all exhibit an increase in rDS(ON) with temperature. Usually this is given in the form of a curve of rDS(ON) vs temperature on the datasheet. The worst case rDS(ON) at any elevated junction temperature is determined as follows. First, using the rDS(ON) vs temperature curve for the device, obtain the multiplicative factor at the expected operating junction temperature. Finally multiply the maximum 25oC rDS(ON) rating by the previously determined factor. The third state we should consider is when the switch transitions from "on" to "off" or vice versa. In many solenoid switch applications the major dissipation occurs while the PowerMOS transistor is "on", but turn on and turn off also dissipate power in the transistor. The switching speed of most PowerMOS transistors is so fast that turn on losses are usually very small. An exception is when the drive current available is very very small. Usually this does not occur in the real world. For example the Fairchild RFP70N06 PowerMOS transistor requires
a maximum of 115nC of gate charge to transition from "off" to fully "on". For a gate drive which supplies 1.0mA this would mean that the transition would take less than 115us. This will make a negligible change in the junction temperature of the PowerMOS transistor. Turn-off subjects the PowerMOS transistor to Unclamped Inductive Switching. Modern PowerMOS transistors can withstand this type of stress and give clear ratings in their datasheets to let customers calculate whether or not they are operating within the devices' capability. The energy dissipated in the PowerMOS transistor each time the current is interrupted is:
L I T V DSS 1 E T = ------------------------------------ 1 K In 1 + --- RL K
(EQ. 1.4)
Defining the Problem
The circuit used in most solenoid switch applications is very simple. It simply consists of an inductor and resistance in series with the drain and a gate drive circuit (Figure 2). Analyzing this circuit can lead to some simplifications that will speed design efforts. There are three circuit states that we should analyze. The simplest state is when the PowerMOS transistor is "off", when the gate and source are at the same potential. Under this condition the dissipation in the device is simply the leakage current times the supply voltage VCC. Usually this is negligible. The second state we should consider is when the gate drive is "on". The PowerMOS transistor can best be represented as a series resistor. The current through that resistor is:
V CC I T = --------------------------------R L + r DS(ON) (EQ. 1.1)
The dissipation (PT) in the PowerMOS transistor while the device is "on" is:
P T = ( I T ) r DS(ON)
2
(EQ. 1.2)
See Fairchild Application Note AN-7514. Where:
V BRK V CC K = --------------------------------IT RL
If we make the simplifying assumption that RL >> rDS(ON) this is:
V CC 2 P T = ----------- r DS(ON) RL
(EQ. 1.3)
where rDS(ON) is the worst case resistance of the PowerMOS transistor at its operating junction temperature. PowerMOS
L
Please note that the VBRK used here is the rated breakdown voltage, since that is worst case, rather than the 1.3 x rated breakdown voltage used in Application Note AN-7514.
L RL
+ VGS RG VDD VGS RG VDD
+ -
0V
0V
FIGURE 1. TYPICAL INDUCTIVE SWITCHING CIRCUIT
FIGURE 2. SOLENOID SWITCHING APPLICATION CIRCUIT
2002 Fairchild Semiconductor Corporation
Application Note 7517 Rev. A2
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