This document is an engineering report describing a two-wire (no neutral) Bluetooth low energy (BLE) smart wall switch using LinkSwitch-TN2 LNK3202D. This demo board is intended as a general purpose evaluation platform for LinkSwitch-TN2.
- Input Stage - The input is half-wave rectified by diode D1. Resistor R1 reduces the peak input current which effectively reduces the input RMS current. A value of 30 kΩ was chosen to provide the maximum current reduction that can still deliver the required load power. At cold start-up, large inrush current may be present. Zener diode VR4 bypasses the input resistor R1 during start-up to be able to operate the circuit properly. The Zener voltage rating is chosen to be higher than the voltage drop on R1 at nominal load. Capacitor C2 provides energy storage as well as EMI filtering. Fuse RV1 provides surge protection.
- LinkSwitch-TN2 Circuit Operation - The controller IC, U1, is configured as a non-isolated flyback switch regulator. Flyback was chosen over buck converter due to better power factor performance of the former which essentially reduces the input RMS current. The main flyback rail is formed by the controller U1, transformer T1, diode D2, and capacitor C6. Capacitor C18 helps reduce the output ripple on the 3.8 V output. During normal operation, the IC is powered by the DRAIN (D) pin and charges the BYPASS (BP) pin capacitor C4. The BP pin capacitor, with a value of 100 nF, programs the current limit to increased mode to provide higher power despite of having a very small transformer T1.
- Bias Supply - Transformer T1, D3, and C13 provide 12 V auxiliary supply to externally bias the BP through R3. The value of R3 is tuned to provide the lowest no-load input current by setting the BP current between 80 µA and 100 µA. The bias supply also charges the capacitor C9 through D13 which provides the energy to the relay coils.
- Feedback - Output voltage is sampled by the FEEDBACK (FB) pin through the feedback resistors R4 and R5. Capacitor C5 provides noise filtering.
Low Drop-Out Regulator Block
The 3.3 V regulator consists of C12, U3, and C11. When the relay is OFF, the input to the LDO comes from LinkSwitch-TN2 through D11. When the relay is ON, the supply comes from Q2 regulator via D9 and R11.
Relay Circuit Block
A 12 V, 2-coil, latching relay RL1 from Panasonic (ADW1203HT) was used. Unlike conventional relay, latching type remembers its last state even when the power is gone, similar to that of a regular wall switch.
Another advantage of using a latching relay is lower power consumption. The coils only need to be energized for around 10 ms and as soon as the relay is latched, the supply can be disconnected.
Transistor Q5, R21, R22 drives the relay OFF while Q4, R15, R33 drives the relay ON. Diode D8 and D12 provide protection from inductive kick. Zener diode VR3 prevents damage to the coil in case the voltage exceeds its maximum rating. When the relay is OFF, the coil is energized from LinkSwitch-TN2 via D13. When the relay is ON, it comes from Q2 regulator via D14.
Regulator Circuit Block
When the relay is OFF, LinkSwitch-TN2 provides 3.3 V and 12 V outputs to power the Bluetooth module and the relay, respectively. As soon as the relay turns ON, however, LinkSwitch-TN2 will turn OFF since the input to the device comes from the relay contacts voltage, which is at 0 V. An auxiliary regulator circuit is necessary to supply the power when the relay is ON.
There are several ways to implement the regulator circuit. The most important consideration is thermal performance. Since Q2 is in series with the line, it must handle the load current, which can be as high as 5 A. The op-amp U2 circuit block controls Q2 switching to allow low-power dissipation by fully turning ON the MOSFET once the threshold set by VR1 + 3.3 V is exceeded. The ON duration is set to about 10 ms by tuning the R-C circuit R10 and C10, tuning C8, and using hysteresis on the input comprised of Q7, R14, R41, R42, and R43. Diode D18 is added to prevent the load current from passing through Q2 body diode once the MOSFET turns OFF. The diode has to be rated to handle the load current. R8 is the series gate resistor.