EL3H7U-G Photocoupler Datasheet - 4-Pin SSOP Package - Profile 2.0mm - Isolation 3750Vrms - CTR 100-560% - English Technical Document

1. Product Overview

The EL3H7U-G series represents a family of compact, surface-mount phototransistor photocouplers (optocouplers) designed for reliable signal isolation in modern electronic circuits. These devices provide a crucial function by transferring electrical signals between two isolated circuits using light, thereby preventing high voltages or ground loops in one circuit from affecting or damaging the other.

The core construction consists of a gallium arsenide infrared light-emitting diode (IRED) optically coupled to a silicon NPN phototransistor. Both are encapsulated within a green, halogen-free compound and housed in a 4-pin Small Outline Package (SSOP) with a low profile of 2.0 mm. This package is ideal for space-constrained applications on printed circuit boards (PCBs).

1.1 Core Advantages and Target Market

The primary advantages of the EL3H7U-G series include its high isolation capability, compact form factor, and compliance with international safety and environmental standards. With an isolation voltage (Viso) of 3750 Vrms, it provides robust protection for sensitive circuitry. The halogen-free material composition aligns with environmental regulations like RoHS and REACH. The device is approved by major international safety agencies including UL, cUL, VDE, SEMKO, NEMKO, DEMKO, FIMKO, and CQC, making it suitable for global markets requiring certified components.

The target applications are diverse, focusing on areas where electrical isolation and noise immunity are paramount. Key markets include switch-mode power supplies (SMPS), particularly DC-DC converters, industrial programmable logic controllers (PLCs), telecommunication equipment, and general-purpose signal transmission across circuits with different ground potentials or impedance levels.

2. In-Depth Technical Parameter Analysis

Understanding the absolute maximum ratings and electrical characteristics is essential for reliable circuit design and ensuring the long-term reliability of the photocoupler.

2.1 Absolute Maximum Ratings

These ratings define the stress limits beyond which permanent damage to the device may occur. They are not intended for normal operation.

• Input (LED Side): The forward current (I_F) must not exceed 20 mA. The reverse voltage (V_R) is limited to 5 V, highlighting the need for proper polarity protection if the input could be subjected to reverse bias.
• Output (Phototransistor Side): The collector current (I_C) is rated at 30 mA. The collector-emitter voltage (V_CEO) can withstand up to 60 V, while the emitter-collector voltage (V_ECO) is much lower at 5 V, indicating the asymmetry of the phototransistor's breakdown characteristics.
• Thermal and Isolation: The total device power dissipation (P_TOT) is 200 mW. The isolation voltage (V_ISO) of 3750 Vrms is tested for 1 minute with pins 1-2 and 3-4 shorted together under controlled humidity (40-60% RH). The operating temperature range is specified from -40°C to +125°C.

2.2 Electro-Optical Characteristics

These parameters, typically measured at 25°C, define the device's performance under normal operating conditions.

• Input Characteristics: The forward voltage (V_F) is typically 1.3V at a forward current (I_F) of 1 mA, which is important for driving circuit design. The input capacitance (C_in) is up to 250 pF, which can affect high-frequency switching performance.
• Output Characteristics: The collector-emitter dark current (I_CEO) is very low (max 100 nA at V_CE=20V), representing the leakage current when the LED is off. The collector-emitter saturation voltage (V_CE(sat)) is a maximum of 0.4V under specified test conditions (I_F=3mA, I_C=1.6mA), indicating a low voltage drop when the transistor is fully on.
• Isolation Parameters: The isolation resistance (R_IO) is a minimum of 5 x 10¹⁰ Ω, and the isolation capacitance (C_IO) is a maximum of 1.0 pF. These values are critical for determining the common-mode rejection and high-frequency noise coupling across the isolation barrier.

2.3 Transfer Characteristics and Grading System

The Current Transfer Ratio (CTR) is the most critical parameter for a photocoupler, defined as the ratio of output collector current (I_C) to input LED forward current (I_F), expressed as a percentage: CTR = (I_CF) * 100%.

The EL3H7U-G series uses a CTR grading system to provide designers with consistent performance bins:

• EL3H7U (Standard): CTR range from 50% to 600% at I_F = 0.5 mA, V_CE = 5V.
• EL3H7UA: CTR range from 100% to 200%.
• EL3H7UB: CTR range from 150% to 300%.
• EL3H7UC: CTR range from 200% to 400%.

This grading allows for more precise design, especially in circuits where gain consistency is important, such as in feedback loops of power supplies. The standard part offers the widest range, suitable for general-purpose applications where exact CTR is less critical.

3. Performance Curve Analysis

The datasheet provides several graphs illustrating key performance trends. It is crucial to note that these curves represent typical behavior and are not guaranteed by production testing.

3.1 Forward Current vs. Forward Voltage (Figure 1)

This graph shows the I-V characteristic of the input IRED at different ambient temperatures (-40°C, 25°C, 125°C). The forward voltage (V_F) has a negative temperature coefficient, meaning it decreases as temperature increases for a given current. This is a typical behavior for diodes and must be considered in thermal management and constant-current drive design.

3.2 Collector Current vs. Forward Current (Figure 2) and CTR vs. Forward Current (Figure 3)

Figure 2 plots the output collector current (I_C) against the input LED current (I_F) for two different collector-emitter voltages (V_CE=0.4V and 5V). The relationship is linear at lower currents but shows saturation at higher I_F levels, especially at the lower V_CE. Figure 3 shows the normalized CTR decreasing as I_F increases. This indicates that the device is most efficient (highest CTR) at lower drive currents, typically around the test condition of 0.5 mA.

3.3 Temperature Dependence (Figures 6 & 7)

Figure 6 demonstrates that the collector current (I_C) for a fixed I_F increases with temperature. Figure 7 shows that the normalized CTR peaks around room temperature and decreases at both higher and lower temperatures. This temperature dependence of CTR is a critical design factor. Circuits must be designed to function correctly over the entire specified temperature range, accounting for the variation in gain.

3.4 Switching Characteristics (Figure 9)

The graph for switching time vs. load resistance (R_L) shows that both rise time (t_r) and fall time (t_f) decrease as the load resistance decreases. Faster switching is achieved with smaller load resistors, but this comes at the cost of higher power dissipation in the output stage. The test circuit (Figure 13) defines t_r as the time from 10% to 90% of the output pulse and t_f as from 90% to 10%.

4. Mechanical, Packaging, and Assembly Information

4.1 Pin Configuration and Polarity

The device uses a standard 4-pin SSOP footprint. The pinout is as follows: Pin 1: Anode of the IRED, Pin 2: Cathode of the IRED, Pin 3: Emitter of the phototransistor, Pin 4: Collector of the phototransistor. Correct polarity must be observed during PCB layout and assembly to prevent damage.

4.2 Soldering and Handling Guidelines

The absolute maximum rating for soldering temperature (T_SOL) is 260°C for 10 seconds. This aligns with typical lead-free reflow soldering profiles. Standard IPC/JEDEC J-STD-020 guidelines for moisture-sensitive devices should be followed. The device should be stored in its original moisture-barrier bag with desiccant under controlled conditions and baked before soldering if the bag has been opened or the exposure time limit is exceeded.

5. Ordering Information and Packaging

The part number follows the structure: EL3H7U(X)(Y)-VG.

• X: CTR Rank (A, B, C, or blank for standard grade).
• Y: Tape and reel option (TA, TB, or blank). TA and TB likely refer to different reel sizes or packaging orientations, both containing 5000 units per reel.
• V: Optional VDE approval marking.
• G: Denotes halogen-free material.

Examples: EL3H7UB-TA-VG would be a B-grade CTR device, packaged on a TA tape and reel, with VDE approval and halogen-free material.

6. Application Guidelines and Design Considerations

6.1 Typical Application Circuits

The primary application is signal isolation. A typical circuit involves driving the input LED with a current-limiting resistor from a digital signal source (e.g., a microcontroller GPIO). The output phototransistor can be used in a common-emitter configuration (collector connected to a pull-up resistor, emitter grounded) to produce an inverted output signal, or in an emitter-follower configuration for a non-inverted signal.

6.2 Key Design Considerations

• LED Drive Current: Select I_F based on required switching speed and CTR. Lower I_F offers higher CTR but slower switching. A series resistor must be calculated using R = (V_source - V_F) / I_F.
• Output Load Resistor (R_L): This resistor sets the output voltage swing, switching speed, and power dissipation. A smaller R_L gives faster switching but lower output voltage swing and higher I_C.
• CTR Degradation: The CTR of photocouplers can degrade over time, especially when operated at high temperatures and high LED currents. For long-life designs, derate the operating I_F and ensure adequate thermal management.
• Noise Immunity: For noisy environments, a small bypass capacitor (e.g., 0.1 μF) across the input pins, close to the device, can help. On the output, careful PCB layout to minimize stray capacitance is important for high-speed signals.

7. Technical Comparison and FAQs

7.1 Differentiation from Other Photocouplers

The EL3H7U-G series differentiates itself through its combination of a compact SSOP package, high 3750 Vrms isolation rating, wide -40°C to +125°C operating temperature, and comprehensive international safety certifications. Many competing devices may offer similar CTR or speed but lack the full suite of approvals or the high-temperature capability.

7.2 Frequently Asked Questions (FAQs)

Q: What is the difference between the standard grade and the A/B/C grades?
A: The standard grade has a very wide CTR range (50-600%). The A, B, and C grades are binned into tighter, guaranteed CTR ranges (e.g., 200-400% for C-grade). Use graded parts for designs requiring predictable gain.

Q: Can I use this for AC input signal isolation?
A: Not directly. The input is an IRED, which is a diode and only conducts in one direction. To isolate an AC signal, you would need to first rectify it or use a dedicated AC-input photocoupler.

Q: How do I calculate the maximum data rate?
A>The maximum data rate is limited by the sum of the rise and fall times (t_r + t_f). A rough estimate for a digital signal is Bandwidth ≈ 0.35 / (t_r). With typical t_r of 8 μs, the bandwidth is about 44 kHz. For reliable digital communication, the practical data rate will be lower.

Q: Why is the isolation capacitance important?
A: Low isolation capacitance (C_IO) is crucial for rejecting high-frequency common-mode noise. In applications with fast voltage transients across the isolation barrier (like in motor drives), a high C_IO can couple noise from the primary to the secondary side, potentially causing malfunctions.

8. Operating Principle and Technology Trends

8.1 Fundamental Operating Principle

A photocoupler operates on the principle of electro-optical-electrical conversion. An electrical signal applied to the input side causes the IRED to emit infrared light proportional to the current. This light traverses a transparent isolation barrier within the package. On the output side, the phototransistor detects this light, generating a base current which in turn controls a much larger collector current. The two circuits are electrically isolated, with only optical coupling between them.

8.2 Industry Trends

The trend in photocoupler technology is towards higher speed, lower power consumption, higher integration, and smaller packages. While traditional phototransistor-based devices like the EL3H7U-G are excellent for DC and low-frequency isolation, newer technologies like digital isolators (using CMOS and RF or capacitive coupling) offer significantly higher data rates, lower power, and better timing characteristics. However, photocouplers maintain advantages in high common-mode transient immunity (CMTI), simplicity, and well-established safety certifications for high-voltage isolation, ensuring their continued relevance in power conversion and industrial control applications.