The coil is a crucial component of high-voltage relays because the coil provides the driving force to close the contacts. The current through the coil generates a magnetic field which attracts the moving core to close the contacts, and on the contrary open the contacts. Even though there are several high-voltage relay vendors in the market, such as TE, Panasonic, GIGAVIC, HF, and so on, the driving current requirements of all the relay coils are similar. The current profile can be divided into three phases, as shown in Figure 3-17. The first phase is known as pickup phase, the current needs to be large enough and keep long enough to make sure the relay is closed during the phase. The second phase is the hold phase, where a smaller current is kept to efficiently close the relay and keep the relay closed. The last phase is current fast decay, during this phase the current drops very quickly to quench the contacts opening. Figure 3-17 shows the three phases requirement of the current curve, the actual currents in the pickup phase and hold phase can be PWM signals with maximum and minimum values.

Figure 2-3 Three Phases of the Relay Current Curve

Generally, the relay vendors provide two relay coil types, one is an economized coil with an internal economizer and the other is a non-economized coil that requires external economization. The economized coil integrates an internal economizer with one of several methods such as a two-coil economizer, pulse-width modulation with voltage feedback, and pulse-width modulation with current feedback. Powering the two terminals of the coil alone is sufficient, and the desired current waveform is generated by itself with this internal economizer. The non-economized coil refers to a coil without any internal circuitry, requiring external circuits to produce the desired current waveform.

It is more preferable from the system perspective to use both high-side and low-side switches to drive the relay coil for safety reasons. The coil is always energized and cannot be shut off when a short-circuit failure happens if only the high- or low-side switch is used. The failure is in line with a short circuit to input on the high-side switch and short circuit to ground on the low-side switch. A large current flows through the coil and cannot be switched off, thus the coil can be damaged due to high power dissipation.

Implementing an elaborate design is indispensable to achieve the current profile. Otherwise, the current through the coil reaches the maximum value determined by the applied voltage divided by coil resistance. Generally, the maximum and minimum current for both the pickup phase and the hold phase are stipulated in each specification to provide the proper operation of the relay. Some vendors prefer to stipulate the minimum effective current in each phase. These currents are much smaller than the current determined by the supply voltage and coil resistance. This not only helps save energy consumption, but also extends relay operation lifetime.

Figure 2-4 shows relay driver circuit.

Figure 2-4 Relay Driver Circuit

RY_24V provides power supply to relay coils. The design uses LM74701-Q1 for reverse polarity protection and a low forward voltage drop regulation. TPS4H160-Q1 is used to switch the RY_24V to coils positive terminal. TPS4H160-Q1 provides a current limit of 8 A which is large enough to cover the pickup phase current with a sufficient margin. Version B of TPS4H160-Q1 is used for the load current monitor function feature. For version B, SEL and SEH are two pins to multiplex the shared current-sense function among the four channels. ULN2803C is used to switch the GND to coils negative terminal. ULN2803C consists of eight NPN Darlington pairs that feature high-voltage outputs with common-cathode clamp diodes for switching inductive loads. The collector-current rating of each Darlington pair is 500 mA. The Darlington pairs are connected in parallel for higher current capability up to 4 A.

Here the high-side switch TPS4H160-Q1 acts as ON and OFF control and protects the coil while short-circuit failures happen at the low-side terminal. A freewheeling circuit is optional for TPS4H160-Q1, because the current through the coil must not interrupt suddenly while turning off the high-side switch. Otherwise, there can be a very large voltage spike due to the coil inductance and probably damage the components. Meanwhile, diagnosis features at both high- and low-side terminals are a benefit for BESS applications.