Product Overview Master PoE Power Transformers: This One Article Is All You Need — VOOHU Electronics

In PoE power system design, the selection of PSE and PD controllers often draws the most attention. However, in real engineering, whether the matching between the power transformer and the Lan Transformer is appropriate directly determines power supply efficiency, thermal performance, and long‑term reliability. Different IEEE standards (af/at/bt) correspond to different power levels, current carrying capabilities, and thermal requirements. The transformer’s core material, winding structure, isolation withstand voltage, and DC bias characteristics must strictly match the application conditions. Improper selection can lead to efficiency degradation, excessive temperature rise, or even equipment damage.

VOOHU Electronics provides a complete transformer solution for PoE applications, covering the full range of power levels from af/at/bt, along with verified reference designs and device matching recommendations. This helps engineers strike a balance between power stability and EMC performance, reducing system debugging risks.

What is a PoE Power Transformer?

Definition

A PoE transformer is a core component of a Power over Ethernet (PoE) system. It delivers stable power to remote devices while simultaneously transmitting data over standard Ethernet cables.

Main Components

PSE (Power Sourcing Equipment)
The PSE initiates DC power delivery. Depending on the standard (af/at/bt), it supplies power over the data pairs (1/2, 3/6) or the spare pairs (4/5, 7/8).
Main applications: PoE switches, PoE injectors, PoE fiber optic transceivers.

PD (Powered Device)
The PD passively receives DC power. Through a PD controller and a DC‑DC converter, it converts the 37V–57V DC from the secondary side into the operating voltage required by the device (e.g., 5V, 12V, 3.3V).
Main applications: PoE network cameras, wireless APs, VoIP phones, PoE lighting fixtures.

Main Functions of the PSE Controller

The PSE controller outputs 48V only after detecting that the connected load meets the power supply conditions. This mechanism delivers adequate power based on the device’s actual needs, ensuring stable operation while effectively preventing device damage or performance degradation caused by underpowering or overloading.

Main Functions of the PD Controller

The PD controller supports hot‑swapping, allowing new PD devices to be connected directly without powering down. Once connected, the PD controller automatically configures itself as a standard PoE load and transfers power from the source to the internal circuitry with maximum efficiency, enhancing the flexibility and scalability of the power system.

Working Principle of the PoE Power Transformer

In essence, the Lan Transformer and the power transformer perform their respective roles, jointly achieving data isolation and voltage conversion.

In networking equipment, the common transformers fall into two categories: Lan Transformers and power transformers, each with different locations and functions.

• Lan Transformer is located at the network interface, between the PHY chip and the RJ45 Connector. Its functions are twofold: (1) transmitting high‑frequency data while performing filtering, electrical isolation, and impedance matching; (2) coupling DC power through its internal center tap, enabling simultaneous transmission of DC power and high‑speed data signals.

• Power transformer is located in the DC‑DC stage of the device’s internal power supply circuit. Its core function is energy conversion: when the switch is on, the primary coil stores energy in the magnetic field; when the switch is off, the magnetic energy is released to the secondary coil, ultimately converting it into the low‑voltage DC required by the device (e.g., 5V, 12V).

Correspondence with PSE / PD

In a PoE power system, a PSE device (e.g., a PoE switch) mainly contains a Lan Transformer, which couples 48V DC onto the Ethernet cable (data pairs or spare pairs) while simultaneously transmitting data. A PD device (e.g., a PoE camera), on the other hand, contains both a Lan Transformer and a PoE power transformer: the Lan Transformer extracts 48V DC from the data lines, and the PoE power transformer further steps down 48V to 5V or 12V to power the chip.

PoE Power Supply Standards

PoE power transformers come in different power levels, corresponding to the three main IEEE 802.3 standards. When selecting a transformer, the appropriate standard should be chosen based on the device’s power requirements.

Mid‑Span (Alternative A & B)
Uses the spare pairs (4/5 and 7/8) of the Ethernet cable to transmit DC power. Different frequencies are used for data signals to achieve multiplexing of data transmission and current delivery.

PoE+ (IEEE 802.3at) fully inherits and is compatible with the two wiring schemes of PoE (IEEE 802.3af) without any change.

End‑Span (Alternative A)
Uses the data pairs (1/2 and 3/6) to simultaneously transmit DC power.
The IEEE 802.3bt standard defines the use of all 8 wires of the twisted‑pair cable for voltage output, addressing the market demand for more powerful and efficient PoE capabilities. It achieves:

• Maximum speed compatibility with 10GBASE‑T;

• A minimum of 40W and a maximum of 90W of power available at the PD.

PoE Power Supply Standards Summary

1. PoE Standards and Power Levels

• Type 1 (802.3af)
  Class 1: PSE 4W, PD 3.84W
  Class 2: PSE 7W, PD 6.49W
  Class 3: PSE 15.4W, PD 13W

• Type 2 (802.3at)
  Class 4: PSE 30W, PD 25.5W

• Type 3 (802.3bt)
  Class 5: PSE 45W, PD 40W
  Class 6: PSE 60W, PD 51W

• Type 4 (802.3bt)
  Class 7: PSE 75W, PD 62W
  Class 8: PSE 90W, PD 71.3W (Note: "713 W" in the original diagram is a typo – the standard value is 71.3W)

2. Power Feeding Methods

• Type 1 & Type 2: support only 2‑pair power feeding

• Type 3 & Type 4: support 2‑pair or 4‑pair power feeding

• Class 5 and above (Type 3/4): always use 4‑pair power feeding

Recommended PoE Power Transformers

For different power levels and application scenarios, VOOHU offers six series of PoE power transformers: EP7, EP10, EP13, EFD15, EFD20, and EFD25. They cover the full range of IEEE 802.3af/at/bt standards, meeting diverse power requirements from IP surveillance, wireless APs, industrial gateways, to digital signage.

• EP7 Series (ST07XXWH): Small size, low power. Typical output 5V/1A. Suitable for compact PD devices such as VoIP phones and single‑port PoE injectors.

• EP10 Series (ST10XXWH): Medium power. Typical output 13.5V/0.75A. Suitable for small wireless APs, indoor cameras, and other af/at applications.

• EP13 Series (ST13XXWH): Power increased to 13W–25W, configurable outputs such as 5V/2.6A. Widely used in PTZ cameras and industrial sensors.

• EFD15 Series (WHTS15EFXX): Flat core design with low profile. Typical output 24V/0.54A. Suitable for height‑sensitive industrial gateways and PLCs.

• EFD20 Series (WHTS20EFXX): Supports higher power (up to 100W). Output 12V/1.08A. Adapts to bt‑standard large‑screen digital signage and multi‑port PD devices.

• EFD25 Series (WHTS25EFXX): High‑power model, can output 12V/6A. Meets bt Type 4 (90W+) requirements for in‑vehicle charging piles and AI edge computing nodes.
  
  

Typical Application Scenarios

IP Surveillance (Outdoor PTZ Camera)

• PSE side: port power ≥30W (at standard), supports 100m long distance without power drop.

• PD side: wide UVLO range, surge protection ≥6kV, efficiency >85%.

Wireless AP (High‑Density Deployment)

• PSE side: sufficient total power budget, supports at/bt standard, per‑port independent reset.

• PD side: precise signature resistance, accurate classification current (Class 4 or higher).

VoIP Phone

• PSE side: port power ≥15.4W (af standard), must be compatible with legacy af devices.

• PD side: low UVLO threshold (to work with older PSEs), low standby power.

PoE Lighting

• PSE side: supports bt Type 3/4, Autoclass function.

• PD side: supports short MPS (maintain power signature), fast dimming response.

Digital Signage (Large Screen)

• PSE side: single port ≥60W (bt standard), total power margin ≥30%.

• PD side: DC‑DC efficiency >90%, robust thermal design (e.g., metal housing).

Industrial Gateway / PLC

• PSE side: industrial‑grade wide temperature, enhanced port isolation, high surge immunity.

• PD side: wide input voltage range (expandable to 60V), wide temperature operation.

Frequently Asked Questions (FAQ)

Q1: What are the roles of VOOHU’s PoE power transformers in PSE and PD devices?

In a PSE device (e.g., a PoE switch), VOOHU’s Lan Transformer couples 48V DC onto the Ethernet cable (data pairs or spare pairs), enabling simultaneous data and power transmission. In a PD device (e.g., a PoE camera), VOOHU’s Lan Transformer extracts 48V DC from the data lines, while VOOHU’s PoE power transformer steps down 48V to 5V/12V to power the internal chips. VOOHU provides EP7 to EFD25 series transformers for all PSE/PD scenarios, with customisable turns ratios.

Q2: What are the main differences between IEEE 802.3 af/at/bt standards? How does VOOHU help with selection?

• af (PoE): max 15.4W – suitable for VoIP phones, low‑power sensors.

• at (PoE+): max 30W, backward compatible with af – suitable for PTZ cameras, wireless APs.

• bt (PoE++): max 90W (Type 4), supports 10GBASE‑T – suitable for large‑screen signage, AI edge devices.

VOOHU offers a full range of transformers covering all three standards, with the EFD20/EFD25 series fully supporting the bt standard (90W+ output). For selection, engineers can refer to the power comparison table on the VOOHU website or contact FAE for tailored advice.

Q3: What package series does VOOHU offer for PoE transformers, and which power levels do they support?

VOOHU offers six series:

• EP7 (ST07XXWH): small size, typical 5W – suitable for VoIP phones.

• EP10 (ST10XXWH): medium power (13.5V/0.75A) – for small APs.

• EP13 (ST13XXWH): 13-25W, configurable 5V/2.6A – for PTZ cameras, sensors.

• EFD15 (WHTS15EFXX): low profile, 24V/0.54A – for industrial gateways, PLCs.

• EFD20 (WHTS20EFXX): up to 100W, 12V/1.08A – for bt standard large‑screen devices.

• EFD25 (WHTS25EFXX): high power, 12V/6A – meets bt Type 4 (90W+), for in‑vehicle charging piles, AI edge nodes.

All series support 42–57V wide input voltage and customisable turns ratios.