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• In a Philips steam generator the TEA1721AT is the primary‑side controller of a small fly‑back switched‑mode power supply (SMPS).
• It converts the 230 V/120 V mains into the low‑voltage rails (≈3.3 V, 5 V, 12 V, etc.) that feed the appliance’s micro‑controller, sensors, display, pump driver and safety circuits, while providing extensive protection (OVP, OCP, OTP, brown‑out, soft‑start).
• It is not the device that switches the main heater element; it only powers the electronics that, in turn, drive the heater TRIAC/relay.
Chip family and topology
• TEA1721AT (NXP GreenChip) is a valley‑switching, primary‑sensing fly‑back controller for ≤5 W auxiliary or standby supplies.
• High‑voltage start‑up current source is built in, so the IC starts directly from the rectified mains without a bulky bleeder.
• An external MOSFET (usually 600–800 V) and a small fly‑back transformer form the power stage; secondary regulation uses primary‑side sensing or an opto‑coupler.
Why a steam generator needs it
• The heating element draws hundreds of watts and is switched by discrete mains devices (triac / relay), but the control PCB needs isolated low‑voltage power that must remain on even when the heater is off or the appliance is in standby.
• The TEA1721AT–based SMPS delivers this “housekeeping” power with ≈80 % efficiency and <30 mW standby loss, meeting EU “ErP Lot 6” and similar energy directives.
Protection & supervision offered
• Cycle‑by‑cycle peak‑current limiting.
• Over‑power and open‑loop protection (OLP).
• Output over‑voltage (OVP) and under‑voltage lock‑out (UVLO).
• Thermal shutdown (≈140 °C junction).
• Valley switching and frequency dithering reduce EMI, easing CISPR 14/EN 55014 compliance for household appliances.
Typical power‑up / run sequence in the iron
a) AC reaches the SMPS via a fuse, NTC, and rectifier.
b) HV start‑up current (~0.6 mA) charges the IC VCC capacitor to ~18 V → TEA1721AT begins switching.
c) Once the first few energy packets return from the transformer, an auxiliary winding sustains VCC (10–13 V).
d) Secondary rail(s) (e.g. 5 V) start the main MCU; the MCU then takes control of heater, pump, fan, display, etc.
Failure signatures when the IC or its environment is bad
• Appliance completely dead, no LEDs or beeps.
• VCC capacitor repeatedly charges / discharges (hiccup start‑up sound).
• 5 V rail low or pulsing → MCU resets, random beeps or error codes.
• Scorched primary MOSFET / snubber; blown fuse.
• GreenChip family (TEA17xx, TEA18xx) is still shipped in 2024 for white‑goods auxiliaries; newer variants (TEA2016, TEA2095) add synchronous rectification or GaN support for even lower loss.
• Appliance makers are migrating to single‑package “HV MCU + SMPS” (e.g. i‑MOTION or PI Geronimo) to shrink the PCB, but discrete GreenChip + MCU remains common in irons and kettles because it is proven and inexpensive.
Analogous device in other appliances
• In many LED TVs you will find a TEA1733 or ICE3BR4765 doing exactly the same job—powering the logic while a much larger PFC + LLC stage drives the back‑light.
Why the heater cannot be supplied by TEA1721AT
• Rated output power of TEA1721AT designs is 3‑5 W. The steam boiler needs 1–2 kW, therefore is controlled through a separate mains path, usually an opto‑isolated triac gate or relay governed by the MCU that is, in turn, kept alive by the TEA1721AT.
Mathematical note (primary peak‑current limit)
\[ I{PEAK} = \frac{V{CS_th}}{R{CS}} \]
with \(V{CS_th}\approx 0.5\ \text{V}\); proper resistor choice is critical for reliable protection.
• Work on mains‑connected PCBs must follow IEC 60335 (Safety of household appliances).
• Any repair that bypasses original fusing, Y‑caps, or insulation violates CE certification and can cause electric shock or fire.
Diagnostic steps
Replacement hints
• Package: SO‑8; respect ESD.
• Keep lead length short to minimise EMI; re‑coat with silicone if the original PCB was conformally coated against steam.
• The exact rail voltages and transformer turns ratio differ between Philips models; always consult the specific service manual.
• Some later boards use TEA152x or TEA193x; confirm the marking before ordering spares.
• NXP app‑note AN11122 “Designing a TEA1721 fly‑back supply” – gives full reference design up to 6 W.
• IEC 60335‑1 and ‑2‑3 for safety requirements of irons/steam generators.
• Latest low‑standby GaN offline converters (e.g. PI LinkSwitch‑TPS) as drop‑in modern alternatives.
The TEA1721AT in a Philips steam generator is the small but essential controller that builds the isolated low‑voltage power supply for every electronic subsystem inside the appliance. By efficiently transforming mains AC to stable DC rails and embedding comprehensive protection, it guarantees that the micro‑controller can safely monitor, pump and fire the main heater, while keeping standby losses and EMI within regulatory limits.
