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Introduction
madnessEfficient energy management is one of the main goals in automotive industry Regulating actuators by Pul Width Modulation (PWM) is a widespread means of improving efficiency. There has been an incre-asing penetration of PWM controlled applications like heater blowers, lamps, EPAS. Once a PWM controller is available in the car it could be ud for veral applications.
宵禁令是什么意思Heat dissipation of monostable relay coils is one source of high temperatures in relay boxes, distribution and switching modules. That limits not only the relay performance, but the performance of the whole unit, too. The heat sources could be removed by using latching relays or at least be reduced by u of high resistive coils and / or by applying PWM controlled driver circuits. This application note summarizes key aspects, which have to be taken into account when using PWM strategy for the relay coil driver.
Relay Status
The best way to regulate the relay coil power consumption would be a DC current driver, since the main electrical parameters of a relay (pull-in, pull-through and holding currents) are to a certain extent temperature independent. But relay coils are usually voltage driven. Thus tho characteristics translat
e into the temperature dependent voltages for pull-in, pull-through and holding. The reason is the tem-perature depending resistance of the coil wire material, i.e. copper. Once the relay has pulled through, it keeps its status (armature keeps to its position on the core) unless the coil current falls below the holding current. For shock and vibration resistance there is an additional excess current required, which depends on the relay type, further relay parameters and shock and vibration requirements. PWM controlled drivers regulate the effective applied voltage by changing the duty ratio of DC voltage normally at a given frequency. Inductive systems like relay coils respond in prence of parallel components to a negative going edge with a current decrea.
Figure 1: Current respon to PWM voltage step with parallel diode
This ripple around the effective current depends on the coil induc-tance, coil suppression, PWM freq
avoided
uency, voltage level and duty ratio.It is always recommended to start with 100% PWM duty ratio until the relay pullsthrough and ttles. The necessary time depends on excess voltage, relay type, etc…, but 500ms should be sufficient. Otherwi it will take some time for the relay current to ttle around the effective current.
基础化妆
七月英文In order to warrant a good relay performance with PWM it has to be made sure, that under all circumstances the coil current does not undercut the level of holding current plus the excess current for shock and vibration. Otherwi the armature and the contacts might open. Then the relay has to pull-in and pull-through again to ttle. Repeated opening and closing the armature might cau humming noi. Unintended opening and closing the armature and contacts under load might cau contact welding.
Inductance
Relay coil inductances are in general relatively high, which result in comparatively small current ripples. But the values are not cons-tant and vary strongly within one relay family or one type. The relay coil inductance depends among others on quite a few parameters, which are not under focus in a standard relay manufacturing pro-cess. Furthermore it heavily depends on the coil current (saturation) and status of the relay (armature open or clod).
Coil Suppression
In DC coil drivers coil suppression is done for protecting the relay dri-ver from high coil switch-off voltage peaks. There are veral options for this (e figure 2). For PWM coil drivers suppression is even more crucial, since the coil switch off occurs at PWM frequencies, i.e. up to veral thousand times per cond. Furthermore coil suppression reduces the ripple coil current, and thus the potential for dropping out since the coil current takes longer to decrea. Therefore from this perspective the stronger the suppression the better, i.e. best with parallel diode (upper circle in figure 2). On the other hand this ca is exactly the worst for relay switching capability. For single drivers the best compromi is probably an anti parallel low voltage (3…9VDC) Z-diode (lower circle in figure 2). A Zener diode in parallel to the driver would cau a varying voltage clamp across the relay coil during switch-off due to varying supply voltage.
Figure 2: Relay coil low side driver with coil suppression options
Ubatt
Frequency:
As could be en in figure 3 the higher the frequency the lower is the ripple current. Therefore the effective coil voltage could be chon to be lower with keeping all the other parameters constant. We recom-mend a PWM frequency of minimum 20kHz.
吉米罗恩PWM 12VDC, 558Hz, 10 kHz, 20 kHz 67%, Tamb. 23°C, Tco il: 50°C
恭喜发财 英文
Figure 3: Effect of different PWM frequencies on ripple coil current on Power F relay with parallel diode
pleasuresDuty Ratio
The effective coil voltage is the product of PWM duty ratio and
supply voltage. However the supply voltage varies due to changes in system load (e.g. cranking) and alternator and battery status. There-fore the PWM duty ratio should be regulated according to the supply voltage. A tight regulation would be optimal for efficiency. But slight variation on the supply side would cau a continuous regulation of the PWM duty ratio. Furthermore regulation respon time would need to be faster than 1ms to ensure, that the effective coil voltage does not undercut the required voltage limit.
天津会计从业资格证
Figures 4 and 5 show a PWM concept for a requirement of 8VDC effective coil voltage using a duty ratio regulation with 2VDC steps of the supply voltage.
Figure 4: PWM duty ratio as a function of supply voltage with 2VDC step regulation
Figure 5: Effective coil voltage as function of supply voltage with 2VDC step regulation
Disturbing Nois
The application of PWM voltage across the relay coil caus magne-tostriction of the iron within the relay magnetic system (core/frame/armature). The result is a slight audible noi if the relay was freely suspended. However when the relay soldered or welded onto a rigid lead frame that noi might be amplified. This depends on the lead frame (suspension, dimension, etc.) and the sound propagation and damping within the car. Choosing 20kHz PWM avoids disturbing nois for human beings but might cau problems to animals.
EMC (Electromagnetic Compatibility)
Due to the steep voltage and current edges EMC problems are possible. Therefore electromagnetic compatibility tests of the whole unit are necessary.
R
e l a y p e r
f o r m a n c e n o t w a r r a n t e d
1210864206
8
101214161820
Supply voltage [VDC]
E f f e c t i v e C o i l V o l t a g e [V D C ]
