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A normally-on circuit can wait for an alarm, fault, or other event while drawing very little current by using a depletion-mode MOSFET to control an enhancement-mode power MOSFET. “Zero-power” is a practical shorthand, not literal zero: the cited design relies on low leakage, and its published sub-100-nA figure applies to the two transistors—not necessarily the complete system.
How the normally-on switch works
A depletion-mode MOSFET conducts when its gate-to-source voltage (VGS) is 0 V. Applying sufficient negative gate bias reduces its conduction until it reaches cutoff. That makes it useful as a normally-closed control element: the circuit can remain in its default state without continuously driving the MOSFET gate.
In the Figure 1 circuit described by Advanced Linear Devices (ALD), Q1 is an ALD114904 N-channel depletion-mode MOSFET and Q2 is an IRF7325 P-channel enhancement-mode power MOSFET. When Q1 conducts, the R3 bias path pulls down Q2’s gate, turning on the load. As negative bias on Q1 increases, Q1 conducts less; at cutoff, the bias changes and Q2 turns off. The control device therefore governs a separate power device that switches the load.
What the 2008 example claims—and what it does not
Bob Chao and Linden Harrison’s ALD application article, published by EE Times on July 22, 2008, reports these figures for its particular circuit. They are historical, source-reported design claims, not guarantees for MOSFETs generally or for a modern implementation.
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| Reported characteristic | Figure 1 claim |
|---|---|
| Supply range | About +2.0 V to +9.0 V |
| Switching | Under 100 ns, described as typical on/off switching |
| Output current | Above 3 A |
| Quiescent current | Below 100 nA from the two transistors |
| Q1 on-state resistance | About 5 kΩ in the article’s circuit explanation |
The under-100-nA figure is not a complete-system standby specification. Control and bias circuitry, the load, leakage paths, temperature, and component variation can all affect actual standby current. Likewise, the article’s current and switching claims need to be checked against the circuit conditions and the selected devices’ current documentation before they are used in a design.
Choose the output polarity for the load’s default state
“Normally-on switch” can refer to the control element’s default conduction, while “load normally on” describes the output state. These are not interchangeable. The Figure 1 example uses a P-channel power device and turns the load on while the depletion-mode control transistor conducts. For the article’s normally-on circuit with the load normally off, ALD suggests replacing the IRF7325 P-channel driver with an IRF7313 N-channel device.
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That substitution is an example of a different output configuration, not a drop-in recommendation for every circuit. Confirm the required gate polarity and voltage, load type, supply range, output current, and off-state behavior for the intended application.
A separate option: zero-threshold EPAD devices
ALD’s article also describes a distinct topology using ALD110800 quad and ALD110900 dual zero-threshold EPAD devices, including ALD110900A in its Figure 2 concept. It reports that a zero-threshold device conducts at 0 V gate bias with 1 µA drain current at VDS = 0.1 V, and describes operation from a 1.5 V supply. This is a separate low-voltage approach; those figures should not be read as characteristics of the Figure 1 power-switch circuit.
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When a MOSFET relay replacement is a different problem
TI’s TIDA-01065 is a related MOSFET relay-replacement reference design, but its stated application is an isolated, self-powered AC solid-state relay. It is not automatically an alternative to ALD’s low-voltage normally-on load switch. The relevant comparison depends on the application’s load and supply, required isolation, control polarity and voltage, off-state leakage, and implementation constraints.
TI lists design files, a schematic, BOM, and layout materials for TIDA-01065. Its assembled board is intended for testing and performance validation and is not available for sale. Those resources may help evaluate the AC relay use case, but do not establish suitability for a low-voltage DC load switch.
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Checks before specifying or building the circuit
- Verify present-day component data. The ALD1148xx and ALD1149xx families are described in the 2008 article as low-voltage, precision-matched dual or quad N-channel depletion-mode devices with threshold values from -0.4 V to -3.5 V. Check current manufacturer documentation for device status, electrical limits, package, and availability before specifying ALD114904, IRF7325, or IRF7313.
- Budget the whole standby path. Estimate leakage and bias current for all circuit components and the load under the actual operating conditions; do not use the two-transistor quiescent-current claim as the system total.
- Check control limits and fault behavior. Ensure the available event signal can produce the required gate bias and that the power MOSFET reaches the intended on or off state across the expected supply and load conditions.
- Separate the topology from the part numbers. The 2008 article explains a circuit concept and gives example devices. It does not establish current lifecycle, sourcing, or suitability for a particular design.
What “zero power” means in context
The article contrasts an emergency-light example using an AC relay module—which it says consumes several watts continuously—with a transistor-switch circuit it characterizes as using a few microwatts. These are broad illustrative claims from the vendor-authored article, not controlled independent measurements or universal savings figures. The useful engineering point is to reduce the power needed to hold a normally-on control state; the actual saving depends on the complete implementation.
ALD’s summary puts the intent this way: “Zero power, normally-ON load switch designs utilizing a combination of an ultra-low power depletion-mode MOSFET and an enhancement-mode power MOSFET is one way that can save cost, space and wasted energy.” — Bob Chao and Linden Harrison, Advanced Linear Devices, in the 2008 article.
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Sources
- Bob Chao and Linden Harrison, Advanced Linear Devices, “Reduce power consumption by redesigning normally-on load switches with zero-power MOSFETs,” EE Times, July 22, 2008.
- Texas Instruments TIDA-01065 reference design.
- Advanced Linear Devices official website.
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