ZVN4206AV - Zetex Semiconductors

Application Note 10

Issue 2 March 1996

Automotive Relay Drivers using the ZVN4206AV

David Bradbury

Introduction

The ZVN4206AV is considered to be the

ideal automotive relay driver. It allows

significant simplification and cost

reductions to be made when replacing

standard drivers such as bipolar

Darlington transistors. The following note

explains the popularity of relays as high

current switches in automobiles, some of

theproblemstobesolvedindesigning

driver circuits, a standard driver circuit and

an improved ZVN4206AV based circuit.

The excellent performance of the

ZVN4206AV is demonstrated and circuit

options discussed.

Automotive Relays

Despite advances in intelligent power

semiconductors over the past few years,

relays are still extensively used to operate

high current automotive loads. Their low

conduction resistance, low cost and

excellent fault tolerance make relays hard

to better in harsh automotive

environments. Relay coil currents vary

considerably with application but the

largest class of automotive relays have 150

to 220mA coils. Controlled by standard

logic or similar low power circuits, these

relays need a medium power driver

transistor to interface to them.

Selection of a suitable driver transistor

requires many constraints to be

evaluated. Automotive power supplies

are rarely clean. Normal operating

voltages are from 10 to 15V for normal

running but if the relay must remain

energised during starter operation then

the lower limit can fall to 6V or less.

Commercial emergency starting

equipment can force the battery voltage

beyond 24V so automotive systems

must be designed to withstand this

voltage for up to 3 minutes to meet

present automotive regulations. Load

dump and other high energy supply

voltage transients must also be

withstood. Additional transients are also

generated by the relay coil as its

magnetic field collapses during turn-off.

Wide variation in ambient temperatures

must also be taken into consideration.

Behind the vehicles instrument panel,

ambient operating temperatures can

vary from -40°C to over 85°C, whilst

within the engine compartment,

temperatures can be even higher.

Meeting these constraints usually

results in specifying an oversized relay

driver or one requiring many protection

components.

AN 10 - 1

Protection Free Load Interfacing with Avalanche Rated MOSFETs

Application Note 10

Issue 2 March 1996

Standard Drivers

Bipolar Darlingtons are currently the

most popular automotive relay drivers

as they have the current gain necessary

to interface between logic and relay with

a single stage circuit. Figure 1 shows a

typical Darlington driver with the extra

protection components needed to

ensure reliability. Diodes provide

reverse supply protection and a clamp

for the relays turn-off transient. A power

VDR is used to limit positive transients

to within the Darlingtons breakdown

voltage. The saturation voltage of the

Darlington (typically over 1V) causes

sufficient power dissipation to eliminate

the safe use of inexpensive TO92 types

(unless they have a 200°C temperature

rating - see Note 1 below) so TO126 or

TO220 types are often selected. The

resulting circuit is expensive, bulky,

awkward to assemble and can be subject

to mechanical reliability problems.

[Note1:

The Zetex E-Line package allows

operation to 200°C, and the product

range includes Automotive specific

Darlington devices.]

The ZVN4206AV Relay Driver

The introduction of the ZVN4206AV

provides a far simpler, if not the simplest

possible solution to the problems of

relay drivers. It is an avalanche rated 60V

1Ω N-channel MOSFET designed

specifically for use with automotive

relays. The ZVN4206AV can drive relays

with coil currents up to 600mA, and

doesnt require a catch-diode to clip the

turn-off transient caused as the relay is

deactivated. The energy stored in the

relays magnetic field is dissipated

harmlessly by avalanche breakdown of

the ZVN4206AVs body diode. The same

body diode also protects the MOSFET in

the event of reverse battery connection.

Characterised for 5V gate drive it can be

driven directly from standard logic with

no interface components.

Figure 2 shows how simple a relay driver

circuit can be made by using the

ZVN4206AV. The devices MOS gate

input requires no DC input current so

direct connection to logic is possible

without buffers or current setting

resistors.

When on, the typical relay coil current of

200mA will cause less than 40mW

dissipation in the MOSFET at 25°C

ambient. Giving an on-voltage drop of

only 200mV at this current, the

ZVN4206AV will operate the relay with

battery voltages 1V lower than can be

achieved with Darlington based

circuits.

Approximately 2mJ is stored in the

magnetic field of the relay whilst

energised. When the relay is turned off,

the current flowing in the relays coil

causes the drain voltage of the

ZVN4206AV to rise rapidly up to the

breakdown voltage of the transistor

(approximately 80V) and the stored

energy is dumped harmlessly into the

body diode of the MOSFET. Figure 3

shows the turn-off voltage and current

waveforms of a typical relay driver

circuit.

If a commercial starting aid is used

which doubles the battery voltage, the

ZVN4206AV will be forced to pass twice

its normal operating current. For loads

of 300mA or less it will stand this

indefinitely - more than can be said for

the relay.

Reverse connected supplies will cause

the body diode of the MOSFET to be

forward biased and hence conduct. The

current will be restricted by the relays

coil resistance to a safe level though the

relay will be energised. With bipolar

driver transistors, harm can come to the

control logic due to a possible current

path from a reverse connected battery

through the driver to the logics output.

This cannot occur with the ZVN4206AV.

Figure 4 shows a load dump supply

transient, caused by the vehicles

alternator if a battery connection should

fail during heavy charging. This

transient could occur when the relay is

on or off and the stress placed on the

driver circuit can therefore be very

different. If off, the 65V transient will not

reach the breakdown voltage of the

ZVN4206AV and so no current will flow.

AN 10 - 3AN 10 - 2

Logic R1

RelayD1

+5V

VDR1

+12V

Figure 1

Darlington Driver with Typical Protection

Components.

Logic

ZVN

4206AV

Relay

+5V

+12V

Figure 2

Reduced Component Count Relay Driver

using the ZVN4206AV.

Figure 3

VDS & IDS of ZVN4206AV During Turn-Off

of Relay.

Figure 4

Load Dump Transient Waveform.

Application Note 10

Issue 2 March 1996

Application Note 10

Issue 2 March 1996

Figure 5 shows the current and voltage

waveforms that occur should the relay

be energised during a load dump

transient. By integrating the

current-voltage product, it can be

calculated that a junction temperature

rise of approximately 20°C is caused by

the transient but this will quickly decay

away. Consequently, infrequent load

dump transients cause no problems for

the ZVN4206AV relay driver circuit.

Figure 6 shows a second transient that is

often used to test automotive electrical

systems - a 350µs time-constant 10 ohm

source impedance inductive discharge

transient. The peak voltage used in this

test varies from manufacturer to

manufacturer over the range 100 to

240V. To give worst-case results a 240V

peak transient is used here.

Figure 7 shows the driver circuits

response to this transient when the relay

is on. The relay coil current rises from its

nominal 200mA to a peak of around

900mA and then falls back. The

ZVN4206AV maintains a low

drain-source voltage for the duration of

the transient so the power dissipation

caused is negligible.

Figure 8 shows the circuits response to

the 240V transient when the transistor

was off before the transient. For this

case, the transient forces the

ZVN4206AV into avalanche breakdown.

The peak avalanche current that flows is

limited by the relays inductance to

around 200mA. The energy dumped into

the transistor, calculated by integrating

the current-voltage product is

approximately 5mJ. These stress levels

are well within the avalanche

capabilities of the ZVN4206AV.

The effect of temperature on the driver

circuit should be considered for

automotive applications due to their

wide operating range. The on-resistance

of the ZVN4206AV increases with

temperature by about 0.65%/°C and this

does reduce its current rating at high

temperatures. However it should be

remembered that the resistance of the

relays coil also increases with

temperature, reducing the driver

transistors load current at the rate of

0.43%/°C. Since the power dissipation of

the ZVN4206AV is I2R related, even

though the MOSFETs resistance

increases with temperature, its overall

power dissipation falls as the I2 factor is

more significant. As a result, a

ZVN4206AV driving a 200mA relay will

operate within its chip temperature

ratings for ambients beyond 140°C.

The use of a ZVN4206AV has given a

circuit capable of withstanding all

commonly applied automotive

transients and conditions of misuse

whilst eliminating the need for bulky and

expensive driver and protection

components. This should not only

provide worthwhile cost savings over

standard drivers but also lead to

improvements in reliability due to

reduced power dissipation and

component count.

Fault Tolerant Relay Drivers

Many automotive relays are mounted in

sockets, remote from their drivers.

Attempts to clean socket contacts during

servicing or fault finding may lead to

intermittent shorts being applied to the

relay driver and so sometimes this must

be catered for. The circuit in Figure 9

shows a ZVN4206AV relay driver circuit

which includes load current sensing. In

the event of the load current exceeding

300mA, the ZTX108 is turned on,

indicating a fault to the controlling

microprocessor. The power dissipated

in the ZVN4206AV (12V x 1.5A) - 18W can

be withstood safely for around 50ms,

plenty of time for the microprocessor to

recognise the problem and turn off the

driver.

AN 10 - 5AN 10 - 4

Figure 5

VDS & IDS During a Load

Dump Transient.

Figure 7

VDS & IDS During a 240V Inductive Line

Transient (Device On Before Transient).

Figure 6

240V Inductive Line Transient Waveform.

Figure 8

VDS & IDS During a 240V Inductive Line

Transient (Device Off Before Transient).

Application Note 10

Issue 2 March 1996

Application Note 10

Issue 2 March 1996

Figure 5 shows the current and voltage

waveforms that occur should the relay

be energised during a load dump

transient. By integrating the

current-voltage product, it can be

calculated that a junction temperature

rise of approximately 20°C is caused by

the transient but this will quickly decay

away. Consequently, infrequent load

dump transients cause no problems for

the ZVN4206AV relay driver circuit.

Figure 6 shows a second transient that is

often used to test automotive electrical

systems - a 350µs time-constant 10 ohm

source impedance inductive discharge

transient. The peak voltage used in this

test varies from manufacturer to

manufacturer over the range 100 to

240V. To give worst-case results a 240V

peak transient is used here.

Figure 7 shows the driver circuits

response to this transient when the relay

is on. The relay coil current rises from its

nominal 200mA to a peak of around

900mA and then falls back. The

ZVN4206AV maintains a low

drain-source voltage for the duration of

the transient so the power dissipation

caused is negligible.

Figure 8 shows the circuits response to

the 240V transient when the transistor

was off before the transient. For this

case, the transient forces the

ZVN4206AV into avalanche breakdown.

The peak avalanche current that flows is

limited by the relays inductance to

around 200mA. The energy dumped into

the transistor, calculated by integrating

the current-voltage product is

approximately 5mJ. These stress levels

are well within the avalanche

capabilities of the ZVN4206AV.

The effect of temperature on the driver

circuit should be considered for

automotive applications due to their

wide operating range. The on-resistance

of the ZVN4206AV increases with

temperature by about 0.65%/°C and this

does reduce its current rating at high

temperatures. However it should be

remembered that the resistance of the

relays coil also increases with

temperature, reducing the driver

transistors load current at the rate of

0.43%/°C. Since the power dissipation of

the ZVN4206AV is I2R related, even

though the MOSFETs resistance

increases with temperature, its overall

power dissipation falls as the I2 factor is

more significant. As a result, a

ZVN4206AV driving a 200mA relay will

operate within its chip temperature

ratings for ambients beyond 140°C.

The use of a ZVN4206AV has given a

circuit capable of withstanding all

commonly applied automotive

transients and conditions of misuse

whilst eliminating the need for bulky and

expensive driver and protection

components. This should not only

provide worthwhile cost savings over

standard drivers but also lead to

improvements in reliability due to

reduced power dissipation and

component count.

Fault Tolerant Relay Drivers

Many automotive relays are mounted in

sockets, remote from their drivers.

Attempts to clean socket contacts during

servicing or fault finding may lead to

intermittent shorts being applied to the

relay driver and so sometimes this must

be catered for. The circuit in Figure 9

shows a ZVN4206AV relay driver circuit

which includes load current sensing. In

the event of the load current exceeding

300mA, the ZTX108 is turned on,

indicating a fault to the controlling

microprocessor. The power dissipated

in the ZVN4206AV (12V x 1.5A) - 18W can

be withstood safely for around 50ms,

plenty of time for the microprocessor to

recognise the problem and turn off the

driver.

AN 10 - 5AN 10 - 4

Figure 5

VDS & IDS During a Load

Dump Transient.

Figure 7

VDS & IDS During a 240V Inductive Line

Transient (Device On Before Transient).

Figure 6

240V Inductive Line Transient Waveform.

Figure 8

VDS & IDS During a 240V Inductive Line

Transient (Device Off Before Transient).

Application Note 10

Issue 2 March 1996

AN 10 - 6

The need for such protection in remote

relay systems can be eliminated by

putting the driver transistor in the relay

module. Figure 10 shows a circuit that

can be used to take advantage of this

technique. The output resistor in the

logic unit protects it against accidental

shorts and the inexpensive

resistor/zener circuit included in the

relay module with the ZVN4206AV gives

ESD and open-circuit drive line

protection.

ZVN

4206AV

2.2

Logic

ZTX

108

+5V

+12V

Remote Relay

Links

Figure 9

ZVN4206AV Relay Driver Circuit with

Overcurrent Protection.

Logic

ZVN

4206AV

Relay

+5V

+12V

BZX84

C22

47K

Link

Relay

Module

Figure10

ZVN4206AV Relay Driver Module.

PARAMETER SYMBOL MIN. MAX. UNIT CONDITIONS.

Drain-Source Breakdown

Voltage BVDSS 60 V ID=1mA, VGS=0V

Gate-Source Threshold Voltage VGS(th) 1.3 3 V ID=1mA, VDS=V

Zero Gate Voltage Drain Current IDSS 10

100 µA

µAVDS=60V, VGS=0V

VDS=48V, VGS=0V, T=125°C(2)

On-State Drain Current ID(on) 3AV

DS=25V, VGS=10V (1)

Static Drain-Source On-State

Resistance RDS(on) 1

1.5 Ω

ΩVGS=10V,ID=1.5A

VGS=5V,ID=500mA (1)

Input Capacitance Ciss 100 pF (2)

Avalanche Current - Repetitive IAR 600 mA

Avalanche Energy - Repetitive EAR 15 mJ

Appendix

Partial Characterisation of the ZVN4206AV. Full characterisation available in the

Through Hole Data Book available from your local Zetex agent.

(1) Measured under pulsed conditions. Pulse width=300µs. Duty cycle ≤2% (2) Sample test.