Cost Efficient Mains Powered Supply Concepts for Wireless Sensor Nodes

Franz Lukasch Institute of Computer Technology

University of Technology Vienna, Austria

lukasch@ict.tuwien.ac.at

Abstract—This paper presents and evaluates different power supply concepts for powering wireless sensor nodes directly from 230VAC mains. The main focus is to find a suitable concept in terms of cost- and power efficiency, which allows for high integration density. The most promising architectures are a capacitive power supply concept and a buck-boost converter concept with an advanced phase-dependent control circuit. Innovative extensions to common solutions are presented and discussed for an output power range of up to 100mW with special respect to light load efficiency.

I. INTRODUCTION One important application in building automation is power

metering and control. This can be done by a large number of wireless sensor nodes which are integrated in electrical appliances to measure and transmit its energy consumption as well as to control the device through its integrated transceiver. Because each node is connected to the mains power most of the time, its power consumption should be in the range of milliwatts which is important especially for a large number of nodes. Hence cost efficiency is another big issue in a large power metering network. The power requirement of such a node was estimated below 100mW when it’s fully active at a supply voltage of 3.3V. These values were estimated by evaluating typical components of a wireless power metering node. Because the sensor node is wireless and does not require direct user contact the power supply doesn’t need to be galvanically isolated which can save costs. A wireless power metering node usually is in standby mode most of the time, therefore the power supply should be efficient especially on light loads. On the other hand due to the high quantity of nodes a topology with a higher integration density may be preferred over a topology with a slightly better power efficiency.

II. TRADITIONAL DESIGN CONSIDERATIONS A linear approach is out of the question because of the

excessive losses as is a conventional transformer because of its size and its idle losses [1]. The two most promising concepts are selected and evaluated for power efficiency, costs and integration density. Enhancements are proposed to better fit the special requirements of a wireless sensor node.

A. Capacitive Power Supply Fig. 1 shows the classic approach of a capacitive power

supply which usually is very reliable [2]. The operation principle is the current limitation through the X2 capacitor C1. The zener diode D1 limits the output voltage of the power supply circuit. While the resistor R2 provides user safety by discharging C1 after the device has been unplugged [3], the resistor R1 limits the maximum inrush current when the device is powered up in the voltage maximum of the mains sine wave. Due to its design this power supply has the best efficiency when the output current equals the through C1 defined input current. When the load draws no current, the energy is dissipated in the zener diode D1 which indicates low efficiency for light loads in respect to the maximum output power. Simulations of the circuit in Fig. 2 for a continuous output power of 100mW show an efficiency of 50% for an output voltage of 3.3V and an efficiency of 90% for an output voltage of 24V. This is because the current has to be higher for lower output voltages to provide the same output power. The higher current directly increases the losses in R1 and the bridge rectifier B1 and indirectly increases the losses in R2. The capacity of C2 has to be larger to allow more current and therefore the value of R2 has to be smaller to maintain the same discharging time [2]. In addition there is a significant amount of design induced reactive input power which is a magnitude higher than the actual effective input power. The main advantages of this design are the low-cost components and the sinusoidal input current which eliminates electromagnetic interference. The main disadvantages are the high reactive input current and the constant power consumption. However, this topology is a good base for further enhancements.

Figure 1. Schematic of a basic capacitive power supply.

This research is funded by the Eurpean Comission within the project SmartCoDe

978-1-4244-9474-3/11/$26.00 ©2011 IEEE 502

B. Buck Converter Another common approach which is widely used is the

buck converter [4] whose schematic is shown in Fig. 2. The most important losses of a buck converter are in the transistor and in the inductor. While the inductor losses usually dominate for higher inductor currents, the switching losses dominate for higher switching frequencies. Because of its topology the efficiency of a buck converter is better for output voltages near the input voltage. In the case of a mains powered sensor node, the difference between input and output voltage is very high which leads to significant problems. To keep the inductors preferably small and cost efficient, their inductance should be minimized. To limit electromagnetic interference the maximum current through L1 has to be limited. Hence the inductor losses will contribute a smaller amount to the total losses and the switching losses will dominate. When switched off, the transistor T1 has to withstand at least 325V (VDS). That requires a physically bigger transistor which is slower due to its bigger output capacitance (drain-source capacitance) which usually dominates compared to the gate-source capacitance. Every time the transistor switches off, it has to charge the output capacity to 325V. The energy which is stored in the output capacity is dissipated when the transistor switches on again. Fig. 3 shows this effect for a commonly used high voltage MOSFET. The losses for each switching cycle are 1,7μJ. Because the dissipated energy on each cycle is C · V2 / 2 where C is the output capacitance of the MOSFET and V is the input voltage into the converter, the lost energy increases dramatically with the voltage. Hence this buck topology cannot be efficient for high voltage differences, especially with high switching frequencies. Fig. 4 shows a simulation of the losses on each switching cycle depending on the drain-source voltage. Because of the nonlinear output capacitance the relation shown in Fig. 4 is not entirely quadratic. A possibility to improve the circuit would be to left out the capacitor C1 which will not only save a high voltage capacitor but also reduce the average input voltage to the buck converter. This is possible because the buck converter is current controlled. The capacitance of C2 has to be high enough to save the energy which is needed during the zero crossing of the voltage. In worst case, all the energy is demanded during zero crossing of the mains sine-wave that would result in an enormous voltage drop which may affect proper operation of the node. Because it’s much more efficient to store energy in capacitors at higher voltages, a higher output voltage is preferred to ensure a small output capacitor C2.

B1 C1

230V

AC1

AC2

Vout

C2D1

L1T1

Vctrl

Figure 2. Schemtic of a basic buck converter.

Figure 3. Charging of the drain-source capacitance of a small power

N-channel MOSFET.

Figure 4. Losses per switching cycle depending on VDS.

III. ANALYSIS AND ENHANCEMENT OF THE CAPACITIVE POWER SUPPLY

As stated above, the capacitive power supply has a much better efficiency for low input currents which leads to high output voltages for a given output power. Hence to generate 3,3V with suitable efficiency a secondary voltage conversion stage is necessary. Because the power supply should be highly integrated and the targeted CMOS technology limit is 24V, the output voltage for the first stage is chosen 24V. At this voltage the energy can be stored in a capacitor and the voltage can be further reduced by a standard high efficiency buck converter. Because of the current limitation all components of the primary stage but the X2 capacitor and the protection resistors can be integrated. Fig. 5 shows the schematic of an enhanced version of the capacitive power supply. The diode rectifier bridge has been substituted by a fully integrated active bridge rectifier (left side), the zener diode has been substituted by an active variable voltage limiter [5] (middle), and an additional current valve has been added to support the bridge rectifier and to prevent current to run back from the output capacitor into the voltage limiter when the voltage is lowered. For the desired energy output of 100mW an X2 capacitor of 68nF is needed in the primary path. Because of the self controlled active rectifier bridge no startup circuit is needed, but a current valve is necessary to prevent energy from flowing back into the MOS bridge.

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Figure 5. Enhanced capacitive power supply.

Through the fully integrated rectifier bridge there is no need of an external diode. Additionally the rectifier losses are greatly diminished. T5 and its control logic are taking over for the zener diode in the basic concept [5]. Because the voltage is tunable through the control logic the voltage can be adapted to the actual power requirements of the load. Usually the voltage is limited to 24V, or T5 is completely closed which sets the power supply into a standby mode in which no effective power is drawn from the mains except for the losses in R1 and R2. In this mode the load draws its energy from the capacitor C2. When the voltage in C2 drops below a specified level the power supply goes to an active state again till the capacitor is replenished. Because of this behave this enhanced capacitive power supply is also efficient for loads with long standby periods as a wireless sensor node, but can provide the necessary power when the node wakes up. T6 and its control logic are necessary to hold the energy in the storage capacitor when T5 is shorted to set the power supply into standby. T6 is also necessary because of the design of the full bridge rectifier. Fig. 6 shows the output voltage of the improved capacitive power supply while supplying a load. Simulations have shown an efficiency of 94% on generating 24V from the mains while drawing 100mW which implies losses of slightly over 6mW. These losses are nearly independent on the load which leads to a suitable low energy consumption during standby periods. Because the majority of the losses occur in the protective resistors R1 and R2 the efficiency is even higher when the supply is dimensioned for a smaller maximum output power due to the smaller input current which will induce smaller losses in R1. Also C1 has to be smaller which will require a higher value for R2 to maintain the same discharge time [3] for the capacitor which will further reduce the losses. Because this stage can produce only higher voltages with an applicable efficiency a classic low voltage buck converter may be used to further reduce the voltage to the target voltage.

0 20 40 60 80 100 120 140 160 180 200

0

4

8

12

16

20

24

VOUT

Time (ms) Figure 6. Output voltage of the enhanced capacitive power supply from

startup.

IV. ANALYSIS AND ENHANCEMENT OF THE BUCK CONVERTER BASED POWER SUPPLY

The problem when a buck converter is used from the mains to produce a small voltage is the high current increase over time due to the high voltage on the inductor. This problem is usually solved by using bigger inductors which will increase the costs, allowing higher inductor currents which will increase the inductor losses and the electromagnetic interferences or by a higher switching frequency which will produce higher switching losses due to the construction of the high voltage MOSFET. An approach to solve these issues especially when dealing with small loads is to use only a small time frame of the input voltage, when it’s near the output voltage. Hence no input capacitor is needed but a buck-boost topology [6] is necessary. Since the voltage maximum of the targeted technology is 24V the usable timeframe is when the rectified mains voltage is under 24V. Fig. 7 shows the input voltage into the sensor node after the high voltage switch. The curve in Fig. 7 is centered on a zero crossing of the rectified input voltage. 235µs before and after the zero crossing the voltage is under 24V. Because the voltage slope is almost constant near the zero crossing (about 100V / ms) and only a small amount after and before the zero crossing is used, the shape appears triangularly. It repeats every 10ms for the 50Hz mains frequency because a bridge rectifier is used. The energy which can be taken out of the mains when using only this voltage range and allowing 50mA of average inductor current (100mA maximum inductor current) calculates to about 27mW. Due to losses the actual possible output power for this case will be lower. For this output power this concept promises a very high efficiency combined with low costs because the small high voltage transistor is inexpensive and even may be integrated together with the switch which controls the electrical appliance which is controlled by the sensor node. Fig. 8 shows the described topology. The slow high voltage switch THV only has to switch twice every 10ms to limit the maximum input voltage. The actual buck/boost switches (T1 – T4) can be fully integrated and have to withstand only 24V of source-drain voltage. Hence they are smaller which leads to a significantly reduced output capacitance, which will reduce losses. On the other hand the output capacitance has to be charged only to 24V instead of 325V when switching the transistor off which will further reduce the switching losses.

Figure 7. Used section of the rectified mains voltage.

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Figure 8. Improved paritally integrated low power mains switched mode

power supply.

A problem with this approach is the output power of only 27mW, which is quite sufficient when the node is in standby mode but may not provide enough power for the short time the node is active. In order to achieve a higher output power the allowed current or the time window may be increased. In order to increase the time window the transistor THV has to ensure that the input voltage never rises above 24V which means that the energy which lies above 24V has to be dissipated in THV. There is a tradeoff between efficiency and the allowed current when the node needs more than 27mW at 24V. The time window and the allowed current are both configurable so that the best configuration dependent on the actual node requirements can be found. Fig. 9 shows the extended time window and the resulting linear losses in THV. When the time window is doubled the energy that can be drawn out triples but there are linear losses during the time where the rectified mains voltage is above 24V, which decreases the efficiency by 25%. The usable energy in that case calculates to 84mW at a maximum inductor current of 100mA. First simulations show an efficiency of up to 90% for the timeframe up to 24V (27mW) but there is still a lot of potential for improvements in the control logic design. The main advantages of this concept are the lack of an expensive X2 capacitor and that there is no reactive input power. On the contrary there has to be a high voltage switch and there will be more electromagnetic interference due to higher input pulse currents. Fig. 10 shows a simulation of the input voltage for a time frame which uses the mains up to 48V.

Vin(V

)

Figure 9. Extended time frame of the input voltage and the resulting linear

losses in THV.

Figure 10. Simulation of the input voltage generation for the following

buck/boost converter.

V. CONCLUSION Both proposed approaches are suitable of powering a low

power wireless sensor node. The capacitive approach has the advantage of very good electromagnetic compatibility but a relatively high reactive input power on the contrary. It needs an external X2 capacitor but can use a fully integrated rectifier bridge while the inductive approach needs a high voltage switch and an external rectifier bridge. Both concepts have a better efficiency for higher output voltages which makes a second stage necessary to get from 24V to the actual node supply voltage (3,3V). Because of that both concepts will need an inductor to convert 24V into 3,3V. For the inductive approach the same inductor which is used to generate the 24V can also be used to convert it down to 3,3V. The efficiency of both converters is strongly dependent on the actual energy profile of the wireless sensor node and the control logic of the converters. In terms of efficiency the capacitive approach may be more suitable but it uses a big X2 capacitor and draws reactive power from the mains. The high voltage switch of the inductive approach may be integrated together with the switch that will control the electrical appliance which is attached to the node. It is also smaller than the X2 capacitor which is another advantage of this concept.

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