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Green design improves normally-on switches

Posted: 17 Nov 2008 ?? ?Print Version ?Bookmark and Share

Keywords:switch design? green trend? power supply?

As circuit power supply voltages decrease and green energy trends grow in popularity, designers should reexamine some of the circuits that continually consume power, to reduce overall system power use. One circuit that requires continuous voltage is the "normally-on" circuit, and it is now possible to redesign this circuit with new components to reduce wasted energy.

Low-voltage operation and ultralow-power use have become increasingly essential as circuit supply voltages decrease. Today's portable CE designs conserve battery power and minimize battery drain by temporarily turning off unused parts of a system, using a high-side load switch.

Load switches are usually processor-controlled and must be CMOS-compatible. Power MOSFETs have become the primary switching element for high-side load-switch applications, and optimum performance is achieved when using a P-channel device. Many of these circuits are known to as "normally-off" circuits as they require the power supply to be on to function.

Increasingly, there is a trend to reexamine high-side load switches as one of the generally accepted types of circuits. Where conventional wisdom and tradition provide a way to build traditional circuits, high-side load switches are wasteful of energy. However, these circuits were tolerated because they only used small amounts of energy at that time.

Minimal-cost power cuts
New green trends call for improvement in the efficiency of these types of applications by many orders of magnitude. Often this can be accomplished at incremental or no additional cost, and economical payback can be achieved across the operating life of the system. Redesign efforts have boosted the energy efficiency of compact fluorescent lamps and small wall-powered modules used for everything from battery chargers and cellphones to printer adapters.

A large group of circuits that work in reverse to these normally-off circuits are known as normally-on circuits. They are required to be on without power and off when actually working or powered. An example is a burglar alarm or an emergency alarm that requires the system to be in a normally-on state. These systems will activate and send a signal only when the normally-on status is interrupted. These types of circuits are continuously on 24hrs a day and normally sit idle, waiting for an event to happen that will disconnect its on status, allowing it to take an action.

This class of circuits is also known as very low-duty cycle circuits, where their off time is very low compared to their on time. For these circuits, the on time power use can become an issue, because they are not doing anything when they are on while waiting for an external event to take place. During the on state, AC or battery power is being used. This is a class of circuits that is normally-on, but requires only very low incidence of off-state.

This situation creates the need for a normally-on load switch, and one that simultaneously consumes zero power. Such a switch needs a depletion-mode MOSFET, which acts like a normally closed spst relay. In the past, the electronics industry has typically designed this type of circuit by using more power from an AC source to build a circuit that performs and emulates the required function.

Normally-off example
An emergency-light design provides an example of a normally-off circuit that continually uses power. Typically, an AC-power module is always connected to an AC power source and burns a small amount of AC power. This power is supplied to a circuit that may employ a normally-closed spst relay. This relay is energized continuously until the AC power source is interrupted, then the spst relay is opened and switched to a backup battery pack to power an emergency light or an alarm.

The AC-power module is always on and must be able to supply sufficient voltage and current to switch this spst relay properly. This usually dissipates several watts of AC power continuously to enable the emergency light module. The actual design requirement is to turn on a power MOSFET to switch on the backup-battery circuit only when the emergency module loses its AC power. The rest of the time, the design wants to burn zero power. This is when a depletion-mode MOSFET and an enhancement-mode power MOSFET can work together to achieve zero power and robust current-driving load switch.

A depletion-mode MOSFET's channel is conductive, and its drain current ID(ON) flows when the gate terminal is at 0V (VGS = 0V), i.e. it requires no gate voltage to function. VGS can even be several volts positive. Applying an increasingly negative bias at the gate reduces conduction in the channel until finally its threshold voltage (-VTH) is reached, and conduction ceases. Depletion-mode electrically programmable analog device (EPAD) products such as the ALD1148xx and ALD1149xx series are low-voltage, precision-matched dual/quad N-channel devices with ultralow threshold voltages categorized in several values over a range from -0.4V to -3.5V.

Achieving zero-power use
The circuit shown in Figure 1 operates so that with 0V on the gate of Q1 (ALD114904), the device is on.

Its on-state resistance Rds(on) is about 5k, which along with a bias resistor R3 forms a resistive divider. As a result, the gate of the P-channel power MOSFET Q2 is pulled low, turning it on and activating the load. If Q1's gate is now pulled increasingly negative, it will reduce conduction until finally -V(TH) is reached. This will cause the gate of Q2 to go high and switch off, deactivating the load.

Figure 1: Shown is a normally-on high-side switch that achieves zero-power consumption using an electrically programmable analog device depletion-mode MOSFET.

The supply voltage for this low-voltage circuit usually ranges from 2V to 9V, and will typically switch on/off in less than 100ns. This load switch can easily source greater than 3A of output current if needed. Because the quiescent current for the circuit results only from leakage currents of the two transistors (less than 100nA), it can be considered to consume zero power.

Since the ALD and the International Rectifier devices in the circuit are both "duals," a normally-on, double-pole version of this circuit can be easily created. The input terminals to the circuit can either be connected together to provide a dpst action, or left independent to form two spst switches.

Normally-on example
To create a normally-on circuit with zero power and the load to be normally-off, replace the P-channel load driver IRF7325 with an N-channel load driver such as IRF7313. In this case, the circuit is normally-on and the load is normally-off, and the entire circuit only consumes leakage currents. Compared to a relay-activated circuit, quiescent power use in this circuit is down by several orders of magnitude, from a few watts to a few microwatts.

A unique normally-on, high-side spst switch circuit is seen in Figure 2, which shows an ultralow-power switch using an ALD110900A.

Figure 2: A micropower normally-on high-side load switch achieves zero-power consumption.

The ALD110800 (quad) and ALD110900 (dual) EPADs have true zero-threshold voltage. This makes them both enhancement-mode and depletion-mode devices. When the gate is set at 0V, the drain current = 1A at VDS = 0.1V, which provides a type of circuit with an output-voltage level that is biased at or near the input-voltage level, but without any voltage-level shift. Here, the zero-threshold transistor Q2 is normally conducting when its gate is high. When enhancement-mode EPAD transistor Q1's gate is made high, its drain goes low, thus turning off Q2 and de-energizing the load. Note the 1.5V supply is below even the threshold voltage of today's MOSFETs.

Increasingly, electronic designers realize that a small amount of wasted power multiplied millions of times in circuits that run 24hrs a day contributes greatly to wasted energy. Drastically reducing energy waste can be achieved by adopting new circuits that use some of the latest components. Zero power, normally-on load switch designs using a combination of an ultralow-power depletion-mode MOSFET and an enhancement-mode power MOSFET can save cost, space and wasted energy.

- Bob Chao
Founder and CEO

- Linden Harrison
Analog Circuit Designer

Advanced Linear Devices Inc.

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