Power electronics symbols are standardized schematic notations—governed primarily by IEEE 315 in North America and IEC 60617 globally—used to represent high-power semiconductor switches like IGBTs, MOSFETs, and SCRs. In practice, these symbols dictate how a low-power control terminal (gate or base) manages high-current flow between main power terminals (collector-emitter or drain-source). Understanding these symbols is critical for reading variable frequency drive (VFD) schematics, designing switch-mode power supplies (SMPS), and troubleshooting inverter welders.
The Complete Power Electronics Symbol Reference Table
The table below maps the most common power switching devices to their standard schematic representations. Use this as a quick-reference when reading OEM service manuals or designing your own gate-drive circuits.
| Device Type | IEEE 315 / ANSI Symbol Traits | IEC 60617 Symbol Traits | Standard Terminals | Common Benchmark Part |
|---|---|---|---|---|
| Power MOSFET (N-Ch) | Arrow on source pointing inward; solid gate line. | Often enclosed in a dashed box; arrow on source. | Gate (G), Drain (D), Source (S) | IRFP460 (TO-247) |
| Power MOSFET (P-Ch) | Arrow on source pointing outward. | Enclosed box; outward arrow. | Gate (G), Drain (D), Source (S) | IXTK66P02P |
| IGBT (N-Ch) | MOSFET gate structure combined with BJT collector/emitter; arrow on emitter. | Similar to IEEE, sometimes with added isolation box. | Gate (G), Collector (C), Emitter (E) | FGA25N120ANTD |
| SCR / Thyristor | Standard diode symbol with a gate lead extending from the cathode junction. | Identical to IEEE; rectangle variant sometimes used for high-power modules. | Anode (A), Cathode (K), Gate (G) | BT152 (TO-220) |
| Triac | Two inverse-parallel SCRs sharing a single gate terminal. | Two overlapping diode triangles with bidirectional arrows. | Main Terminal 1 (MT1), Main Terminal 2 (MT2), Gate (G) | BTA16-600BW |
| Power Diode | Triangle pointing to a bar; cathode bar is solid. | Identical, but may include a circle enclosure for high-voltage rectifiers. | Anode (A), Cathode (K) | RURG3060CC (Hyperfast) |
Regional Standards and the 'Rows People Get Wrong'
While basic passive symbols are largely universal, power semiconductor symbols diverge based on regional drafting standards. In the US, schematics typically follow IEEE 315 (formerly ANSI Y32.2). In Europe and most international markets, IEC 60617 is the legal and practical standard. The primary difference is structural: IEC heavily favors enclosing semiconductor symbols in rectangular boxes to denote physical packaging or isolation boundaries, whereas IEEE draws the raw junction topology.
The Rows People Get Wrong
When reading or drafting power schematics, hobbyists and junior technicians frequently misinterpret these specific symbols:
- IGBT vs. MOSFET Terminals: People often draw 'Drain' and 'Source' on an IGBT symbol. This is incorrect. An IGBT has a MOSFET input (Gate) but a Bipolar Junction Transistor (BJT) output. Therefore, the output terminals are always Collector and Emitter. If you see Drain/Source, you are looking at a MOSFET, which has vastly different gate-charge and tail-current characteristics.
- SCR vs. Triac Gate Referencing: A Triac symbol looks like two SCRs back-to-back, but the gate terminal is strictly referenced to MT1 (Main Terminal 1), not MT2. Triggering a Triac requires a voltage differential between G and MT1. Wiring the gate drive relative to MT2 based on a misread schematic will result in a dead short or a non-firing device.
- N-Channel vs. P-Channel Arrows: The arrow on a MOSFET or IGBT symbol indicates the direction of conventional current flow across the body diode PN junction, not electron flow. For an N-channel device, the arrow points in (from P-type body to N-type channel). For a P-channel, it points out.
Safe Interpretation When PCB Markings Are Faded
When you are troubleshooting a blown inverter board and the PCB silkscreen is burned away or the component's laser etching is charred, you must identify the device electrically. According to Fluke's transistor testing guidelines, you can differentiate the three main power switches using a standard digital multimeter (DMM) in diode-test mode.
- Isolate the Component: Desolder at least two of the three pins from the PCB to prevent parallel circuit paths from skewing your readings.
- Test the Control Terminal (Gate/Base): Place your DMM probes between the suspected Gate and the other two terminals.
- If you read OL (Open Loop) in both polarities on both pins, the gate is isolated. You are holding a MOSFET or IGBT.
- If you read a 0.5V to 0.8V diode drop in one direction between the Gate and one of the main terminals, the gate is a physical PN junction. You are holding an SCR or Triac.
- Differentiate MOSFET vs. IGBT: If the gate is isolated, check the main terminals. A power MOSFET will typically show a body diode drop (0.4V - 0.7V) between Drain and Source in one direction. An IGBT will also show a diode drop, but high-voltage IGBTs (like the FGA25N120) often have a much higher forward voltage drop on their internal anti-parallel diode compared to a standard MOSFET body diode. For absolute certainty, check the Electronics Tutorials IGBT guide to cross-reference the package pinout (e.g., standard TO-247 pin 1 is almost always the Gate).
Frequently Asked Questions
What is the difference between an IGBT and a MOSFET symbol?
Visually, the input side (Gate) of both symbols looks identical, featuring an isolated line representing the capacitive gate oxide. The difference lies in the output side. A MOSFET symbol uses 'Drain' and 'Source' with an arrow on the source lead indicating the body diode. An IGBT symbol replaces the Drain/Source with 'Collector' and 'Emitter', reflecting its BJT output stage. In practice, this means an IGBT symbol tells the designer to expect a fixed Vce(sat) voltage drop during conduction, whereas a MOSFET symbol implies an I²R resistive drop.
How do you read power electronics symbols on a faded PCB silkscreen?
When silkscreen designators (like Q1, Q2, or SCR1) are burned off, rely on the physical circuit topology rather than the symbol itself. Trace the copper pours. The terminal connected to the massive ground plane or negative DC bus is almost always the Source (MOSFET), Emitter (IGBT), or MT1/Cathode (Triac/SCR). The terminal connected to the high-voltage transformer primary or motor phase is the Drain, Collector, or MT2. The thinnest trace, usually routed through a gate-drive resistor (typically 10Ω to 100Ω), is the Gate.
Why do European and American power schematics use different diode symbols?
The divergence stems from the philosophical differences between IEEE 315 and IEC 60617. American IEEE standards prioritize drawing the internal semiconductor physics—showing the P-N junctions, the gate oxide, and the exact terminal connections. European IEC standards prioritize the 'black box' functional approach. Under IEC 60617, complex power modules (like a 6-pack IGBT inverter bridge) are often drawn as a single large rectangle with standardized pin numbers and logic gates inside, rather than drawing out all six individual transistor symbols and their freewheeling diodes. Always check the title block of the schematic to see which standard the drafting engineer used.
